Tracy Porter, the Broncos’ starting right cornerback, departed Sunday’s loss to Houston slightly before the end of the first half. He did not return.

Neither did he disappear into the locker room to get urgently-needed medical attention. Every time my binoculars found him on the Broncos sideline, he was sitting on the bench or standing and watching the action.

Perhaps he was injured, as head coach John Fox said afterward. “Knee,” Fox said by way of explanation, and Porter was indeed limping as he walked off the field at the end of a 31-25 home loss, although he was still in his uniform pants and there was no evidence of ice or any other treatment during the intervening two hours or so.

Normally, there’s little doubt about injuries because we see them take place or the club announces them in the press box or both. Neither occurred in the case of Porter. The Broncos announced injuries to linebacker Nate Irving and running back Willis McGahee during the game, but made no mention of an injury to Porter, leading to the conclusion that he he’d been benched.

After all, he came out of the lineup after Texans quarterback Matt Schaub completed a pair of long touchdown bombs to receivers Porter was covering. Andre Johnson caught a 60-yard scoring pass in the middle of the first quarter to give the Texans their first lead at 7-5 and Kevin Walter caught a 52-yarder in the middle of the second to make it 21-5. These were the biggest plays of the afternoon.

“They challenged us, played a lot of man coverage,” Texans head coach and former Broncos quarterback and offensive coordinator Gary Kubiak said afterward. “Jack got very aggressive in some of the things he did, so I tried to give us chances to make big plays, and we did.”

He was referring, of course, to Jack Del Rio, the Broncos’ first-year defensive coordinator, long known for aggressive defensive schemes.

“Andre makes a big play on the boot throwback early in the game,” Kubiak continued. “The throw that Matt made to Kevin for a touchdown was a tremendous play. But we knew we were going to have to make some big plays. It wasn’t a percentage-type throwing day because of the way they played us. But it was a big-play throwing day, so we were able to make those plays.”

Porter was the AFC defensive player of Week 1, largely for the pick six that sealed the Broncos’ opening night victory over the Steelers. Del Rio made it clear that night that he was not flipping the coverage to keep Champ Bailey on the opponent’s top receiver, which the Broncos did routinely before Del Rio’s arrival. For one week, anyway, it didn’t hurt them.

Sunday, it did. Once Porter went out, nickel back Chris Harris took his place as the second cornerback. Tony Carter moved up to nickel back.

At that point, Del Rio allowed Bailey to return to the old formula. He generally shadowed Johnson for the rest of the game, shutting him down without a catch until Schaub found him on a short out with just over two minutes left in the game.

So I asked Bailey afterward if, given a choice, he would cover the opponent’s top receiver all over the field, wherever he lines up.

“I really just do what my coaches game plan for the week,” said the 11-time Pro Bowl selection. “I think everybody in the world knows I always want the best guy. I’ve never been shy about saying it. It’s really their call. I can’t just go against them.”

So there’s the challenge for the Broncos’ defensive coaches. If Del Rio wants to play aggressive defenses that leave his corners on their own, perhaps he should take some advice from one of the best to play the position.

“As a corner, that’s one thing you just don’t want to do, is give up the deep one,” Bailey said. “They could throw a hundred comebacks or curls, but don’t give up the deep one. It’s tough out there on that island, I’m telling you, but it is what it is. We’ve got to learn from it and try to get better.”

On the bomb to Johnson, it looked as if Porter was expecting help over the top from safety Mike Adams. The touchdown throw to Walter looked like basic man-to-man coverage.

Adams offered no insight: “I got to go back to the film and see what happened,” he said. “I just saw the (still) pictures and that didn’t help me much.”

The last time a cast wanted to see the film as much as these Broncos, Francis Ford Coppola was making the original Godfather.

“You never really feel like somebody’s better than you,” Bailey said. “It’s just we’re killing ourselves because we know what they’re going to do but our eyes aren’t in the right place, and that makes you look bad. That’s how they make plays. I mean, their offense is set up off that run game and if you don’t stop the run effectively they can eat you up in the boots and play-action and stuff like that.”

That’s what happened on the first-quarter touchdown to Johnson.

“It was just a double move,” Johnson explained. “They had the perfect coverage. There wasn’t anybody on the other side of the field.”

The Broncos thought the rebuilt back end of their defense was good enough to play man coverage and let Del Rio play the run and go after the quarterback with everybody else. So far, it looks like they were wrong.

Schaub threw four touchdown passes against Del Rio’s defense. That’s a lot. The Broncos’ front seven was victimized by the zone blocking, cut blocking scheme other teams complained about for many years when the Broncos ran it under Mike Shanahan, Kubiak’s former boss.

“You try to practice and prepare for it as much as you can, but you can’t practice (cut-blocking),” Broncos rookie defensive lineman Derek Wolfe said. “You can’t practice the back cutting on you like that. So that was definitely something new for us. I thought we handled it well at times, but there were just some misfits here and there. We got out-schemed, I think.”

Schaub’s shortest touchdown flip was a three-yard swing pass to running back Arian Foster. Somehow, 330-pound defensive tackle Kevin Vickerson seemed to have coverage responsibility. I have no idea what scheme that is, but it may need further review.

Schaub also had a scoring strike to Owen Daniels. Covering tight ends has not been the Broncos’ forte through the season’s first three weeks.

And then there were the two big plays. Kubiak denied he was going after Porter.

“I wouldn’t say we target anybody,” he said. “We’re attacking scheme, attacking what they’re doing on the back end, whether it’s quarters, quarter-quarter-half, or man. But we knew that we would have to try to get the ball down the field because they had a lot of people committed to the run. We just came off a game where we played a team that played us in a bunch of two-deep and we had to play the game totally different. So we did what we had to do to win. Convinced them to make some big plays and they did.”

If this is the sort of defense that Del Rio intends to run — a high-risk, gambling unit that leaves corners on their own — he’d better make sure his best cover corner is covering the other team’s most dangerous weapon.

Yes, Schaub completed a key 12-yard pass for a first down to Johnson with Bailey on him at the end, but that’s still the matchup you want if you’re a Broncos fan.

On the other side of the ball, the Broncos were uninspired and uninspiring for most of the game. Offensive coordinator Mike McCoy seems determined to use the no-huddle as a change of pace, so the Broncos go long stretches huddling conventionally and looking thoroughly mediocre. Then they’ll break into the no-huddle and start moving the ball.

For most of the afternoon, Peyton Manning was clearly the second-best quarterback on the field. Through three quarters, he had completed 17 of 35 passes for 210 yards, no touchdowns and a pedestrian passer rating of 67.6. The one saving grace was he eliminated last week’s interceptions.

In the fourth quarter, against a Texans defense protecting a 20-point lead, he finally connected on a 38-yard touchdown to Brandon Stokley, his old Colts teammate, to cut the lead to 13.

“It was just a seam route,” Stokley said. “Peyton made a great throw right over the top of the guy. I thought we were able to get some stuff going after that. It kind of got our confidence going maybe and the defense started playing well and we were able to claw our way back into the thing.”

Fired up, the Denver defense delivered a rare three-and-out. Freed by the urgency of the no-huddle, the offense marched down the field with Manning leaning heavily on Stokley, the most familiar of his receivers. When his throw for Eric Decker was deflected into the waiting arms of tight end Joel Dreessen in the end zone, it seemed karmic compensation for Demaryius Thomas’ failure to get two feet down on a perfect touchdown throw five plays earlier.

Suddenly, the Broncos were down only six, just as they were a week earlier in Atlanta. Using their timeouts, they forced the Texans into a third-and-five with 2:49 on the clock. Johnson pushed Bailey off him at the line of scrimmage far enough to give him room to break to the outside. Schaub placed the ball perfectly, just beyond Bailey’s outstretched arm.

“We lined up in one formation and shifted to another,” Johnson said. “Champ was playing outside of me and I knew I had an out-breaking route. I started outside and pushed back up and broke out and Matt gave me a chance.

“I went to Matt earlier, before we got the ball, and said, ‘I’ve been playing (badly). Just give me a chance. Don’t give up on me.’ He came to me and said that I’ve been playing too much football to get down on myself. He gave me the opportunity and I was able to make a play.”

Johnson was referring, probably, to a couple of earlier near-misses — a bomb down the right sideline broken up by Bailey that Johnson appeared to catch momentarily with one hand, and another that he almost juggled into Adams’ arms. But it’s instructive that despite his early touchdown, he was frustrated enough at the end to apologize to Schaub before his third-down catch, just his second of the game.

“I was right there,” Bailey said. “It’s just two good players making a play. His quarterback put it right where I couldn’t get it, so I’ve got to give him a lot of credit. Once I started following him around, he didn’t have a catch. In crunch time, he made it happen, so you’ve got to give him a lot of credit for that.”

If an opposing receiver can beat Bailey, the Broncos will have to live with it. He’s the best they have and one of the best there’s ever been.

If, on the other hand, they lose because an opposing No. 1 receiver beats their No. 2 cornerback, as Johnson did in the first quarter, that’s like a pitcher getting beat on his second-best pitch. That’s a mistake.

The Broncos may well need to play the high-risk defense Del Rio called Sunday. They may still not be stout enough up front to shut down the ground game and pressure the quarterback with four down linemen and the occasional linebacker, as the Texans were able to do.

But one lesson of Sunday’s loss seems pretty obvious: If that’s how they’re going to play, they need to let Champ Bailey cover their opponent’s best receiver until somebody else proves he can do it better.