INDIANAPOLIS — Tony Dungy thought the Colts might finally be close to full strength this week.

Not so fast.

As Bob Sanders, Joseph Addai and Kelvin Hayden were all cleared to practice for the second straight day, Dungy announced that Indianapolis' injury rash has hit another starter: cornerback Marlin Jackson, who could miss the rest of the season with a knee injury.

"He did significant damage to the knee, and I'm not exactly sure, but it's probably going to involve some surgery," Dungy said. "I think he might get a second opinion, but right now, he's probably going to be out for a while. Most likely, we're going to put him on injured reserve."

The injury is believed to be to Jackson's right knee, but Dungy could not say that definitively, nor did he provide details of the diagnosis. The NFL's Web site reported Jackson tore the anterior cruciate ligament and meniscus cartilage.

Dungy said Jackson was injured midway through Wednesday's practice, which is closed to the media, when he was running and tried to stop. It was a no-contact practice and the Colts weren't even in pads.

Jackson was not in the locker room Thursday, and teammates said they had not spoken with him.

He was the Colts' first-round pick in 2005 and arguably their most versatile defensive back because he could play both safety, where he started eight games in 2006, and cornerback, where he has made 23 consecutive starts since 2007.

This year, Jackson was third in tackles (57) and had one fumble recovery, and it's an even bigger blow because Indy plays New England and its talented receiving corps Sunday.

Jackson's likely replacement will be Tim Jennings, a 5-foot-8, 185-pound third-year player who has been filling in for the injured Hayden, Indy's other starting cornerback.

"It's part of the game, and we've just got to keep moving," Jennings said. "I'll be ready."

Word of Jackson's injury tainted what was supposed to be a bright day for the Colts.

Sanders went down with a high ankle sprain in Week 2 against Minnesota, then had arthroscopic surgery on his right knee during his six-week absence.

The 2007 Defensive Player of the Year finally returned to practice Wednesday and said Thursday he was ready to play.

"I feel good, and I'm very eager to get back out there," he said. "I just want to be as close to 100 percent as I can, but I don't want to hurt the team."

Most figure getting the Colts' defensive leader back can only help.

Indy has lost two straight, fallen four games behind the AFC South-leading Titans and, some believe, based on past performances, Sanders' return could turn things around. In 2006, he missed 12 games with a knee injury, then returned for the playoffs and solidified a leaky run defense in the Colts' Super Bowl run.

"It's not all on me, even though everyone wants to put it on me like I was the savior of the season," he said. "If you go back and look at the tapes, it was just that everybody did their job. I only do my job, and if I can do that, hopefully, we can get this thing turned around."

Sanders, a two-time Pro Bowler, does more than most. Dungy has nicknamed Sanders "The Eraser" because he covers up other players' mistakes with his aggressive style.

But Hayden may now be the more critical returnee. At 6-foot, he's the tallest remaining player in the secondary — which will be key to defending 6-4 Randy Moss this weekend.

Hayden, who returned an interception for a touchdown to seal the Colts' Super Bowl win, missed three straight games after having surgery to repair a torn meniscus in his left knee.

"I don't think I'm 100 percent, but I think I'm close enough to 100 percent to play," Hayden said before Wednesday's practice.

Sanders, Hayden and Addai, the Pro Bowl running back who has missed two straight games with a hamstring injury, participated in all the drills Wednesday and were expected to be full participants Thursday.

That's a promising sign for a team that's been short-handed since training camp opened.

Two-time league MVP Peyton Manning (knee), former league sacks champion Dwight Freeney (foot) and Sanders (shoulder) all started camp on the physically unable to perform list.

Since then, little has changed.

Pro Bowl center Jeff Saturday missed the first two games with a knee injury; left tackle Tony Ugoh was out of the lineup for four games with a groin injury; rookie guard Mike Pollak was out for a month with a knee injury; and guard Ryan Lilja (knee) is still on the PUP list.

Those were four of the five projected starters along Indy's offensive line.

Pro Bowl receiver Reggie Wayne, who sat out Wednesday's practice with a knee injury, was expected to practice Thursday, Dungy said.

But the loss of Jackson is one more hit the Colts can't afford.

"We obviously feel bad for Marlin, but we have to continue on," Dungy said. "We'll know in a few days exactly what's going on and what's going to be required. For right now, we know he's going to be out for a while and more than likely will go on injured reserve."