By MICHAEL MAROT
AP Sports Writer
INDIANAPOLIS -- Tony Dungy thought the Colts might finally be close to full strength this week.
Not so fast.
As Bob Sanders and Joseph Addai practiced for the second straight day, Dungy announced Thursday that Indianapolis' injury rash has hit another starter: Cornerback Marlin Jackson will 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 before practice. "I think he might get a second opinion, but right now, he's probably going to be out for a while."
A few hours later, the Colts placed Jackson on injured reserve and re-signed cornerback Keiwan Ratliff, who has played in four games with Indy this year.
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.
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 Kelvin Hayden, Indy's other starting cornerback.
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, fans hope Sanders' return will turn things around as it has before. 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."
It could get worse. Hayden (knee) was out of Thursday's practice and if he misses his fourth straight game Sunday, the Colts would have no defensive back taller than 5-foot-11 to defend 6-4 Randy Moss.
Addai missed the last two games with a hamstring injury, but practiced both days this week. Pro Bowl receiver Reggie Wayne, who Dungy thought would practice Thursday, did not work.
Meaning the Colts could be short-handed as they have seemingly been 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, Pro Bowl center Jeff Saturday, left tackle Tony Ugoh, rookie guard Mike Pollak have all missed games and guard Ryan Lilja (knee) is still on the PUP list. Those were four of the five projected starters along Indy's offensive line.
But the loss of Jackson is another 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."