Bob Sanders isn't healthy enough to play yet. Indianapolis hopes he's back before mid-October.

Yes, the Pro Bowl safety avoided opening this season on the physically unable to perform list, a move that would have kept him out another six weeks and a move that new coach Jim Caldwell acknowledged means Indy anticipates having the 2007 NFL Defensive Player of the Year back sooner.

"The biggest thing was that we felt he (Sanders) was going to be back inside that five- or six-week window," coach Jim Caldwell said Monday. "He's making progress, he's coming along."

Sanders had offseason knee surgery and has been the key cog on the Colts' defense over the past four seasons.

In 2005, when he started 14 games, the Colts opened 13-0. In 2006, Sanders missed 12 regular-season games and the run defense ranked among the league's worst. When Sanders returned for the playoffs, Indy's suddenly staunch run defense was one of the catalysts for their Super Bowl run.

Two years ago, Sanders started 15 games and was named the league's best defensive player, and last year when Sanders made only six regular-season starts, Indy's run defense again ranked in the bottom third of the league.

Clearly, the Colts need a healthy Sanders on the field.

"Obviously, there's a difference with Bob not being out there," said middle linebacker Gary Brackett, the defensive captain. "But our motto has always been 'Next guy up.'"

Caldwell has not officially ruled Sanders out of Sunday's opener against Jacksonville. But it is unlikely he will play. Sanders has not practiced all summer and was not available for interviews Monday.

Three-year veteran Melvin Bullitt, who made nine starts last season, would be Sanders' likely replacement. In 2008, Bullitt finished with 51 tackles and led the Colts with four interceptions _ two of which sealed victories, at Houston and at Pittsburgh.

"Melvin will be out there and (former Pro Bowl safety) Antoine (Bethea) will be out there, and we know what they can do," Brackett said. "I have total confidence in Melvin because he is a professional who is working all the time to get better."

The Colts covered themselves by adding veteran safety Aaron Francisco to the 53-man roster.

Francisco spent the last four seasons with Arizona. He was cut Friday and is expected to make an immediate impact on special teams, though Caldwell acknowledged Francisco must learn Indy's terminology before he plays on defense.

To make room for Francisco, Indy released second-year linebacker Jordan Senn.

"Hopefully, I'll be ready by this week," Francisco said. "It depends how quickly I can digest things, but it sounds like a lot of the things we do here are similar to what we did in Arizona."

The cohesiveness of Indy's defense is another concern.

Injuries limited more than half of Indy's projected starters during the preseason.

Bethea (hand), ends Raheem Brock (hand) and Robert Mathis (thigh), cornerbacks Kelvin Hayden (hamstring) and Jackson (knee), linebacker Clint Session (ankle) all sat out at least one preseason game. The regular starting unit still has not taken a single snap together, though all of those defenders are expected to play Sunday.

Another starter, 296-pound tackle Ed Johnson, will not play Sunday. He is serving a one-game suspension because of last year's arrest on a drug possession charge. The Colts immediately cut Johnson after the arrest and he did not sign with another team until the Colts brought him back in May.

"That's always something we have to kind of work our way through," Caldwell said. "One of the things we have talked about is that performance is what counts. So now we have to put it all together. Some of the pieces weren't there early on but they are now."

Everyone, that is, except Sanders.

So, when?

"How soon that is, I'm not certain," Caldwell said twice. "As soon as I get a bead on that, I'll let you know."