Brackett, the Colts' defensive captain, has a bruised left shoulder. Sims sprained a knee during last week's season-opening loss at Houston.
Brackett told reporters that he still hopes he can play Sunday, according to the Star.
"Well, I can tell you when I said those two guys are out, unless some miracle occurs, OK?" coach Jim Caldwell told reporters Wednesday. "And it's happened with a couple guys.
"That's why often times we kind of avoid even telling you that because of the fact that if they happen to pop up, if he happens to come through and say, 'Coach, you know what, I can go,' then we're going to let him go. And I'll just have to come back and tell you I was wrong."
If Brackett isn't able to go, second-year linebacker Pat Angerer is expected to shift to the middle linebacker position.
Caldwell said outside linebacker Kavell Conner, who suffered a left foot injury in the opener, should be able to play.
The Colts re-signed linebacker Nate Triplett, who was among the team's final cuts.