Freeney suffered the injury in the third quarter of Indy's 31-10 victory over the Cardinals on Sunday.
After the game, Freeney said he "felt something pop" in his leg. He had an MRI exam on Monday.
Colts spokesman Craig Kelley said the team would not provide a more detailed injury report until Wednesday. Freeney's agent, Gary Wichard, did not respond to e-mails sent by The Associated Press.
The injury to the Colts' all-time sack leader is another blow to a defense already missing three starters and still trying to get accustomed to Larry Coyer's revamped system.
The Colts (3-0) have been inconsistent on defense. They played reasonably well against the run and made a late stand to preserve a season-opening win over Jacksonville, then couldn't get off the field in a victory at Miami. Against Arizona, Freeney was the ringleader of a pass rush that constantly harassed two-time MVP Kurt Warner into miscues in what was clearly the defense's best performance this season.
Losing Freeney, though, creates a huge void.
Coyer spent much of the offseason plotting more movement along the defensive line and more blitz packages to take advantage of the Colts' speed and the team's top playmakers, like Freeney.
"I'm still day-to-day, there's a bruise in my knee and right now we're taking what it gives us," Brackett said. "I didn't make the trip because of the flight. This week is another thing, I'm going to try to play. But you have to be smart about it and when you're healthy enough to go out there and really be productive, that's when you go back."
Freeney, the NFL sacks leader in 2004, already has four of Indy's seven sacks this season and leads the team with nine quarterback pressures.
Since spending his first eight NFL games as a backup, Freeney has missed just 12 of the Colts last 99 games -- seven coming in the second half of 2007 when he sustained a season-ending foot injury -- and he has played through a series of nagging injuries during his eight NFL seasons.
"They trust one another and give tremendous effort and it does not matter how many injuries we have, they step in and play well," Caldwell said. "We just expect that next person to step in every single week and do the job. Often times, you find teams that talk about injuries and how they'll be affected by that and I think there's a trickle-down effect where guys get that in their mind and think we're going to have a down game. We don't do that."
The Associated Press contributed to this report.