"He's got a serious recovery that he's got to go through, and it's going to take a while," Carroll said Monday.
Thomas suffered the injury in the second quarter when he collided with teammate Kam Chancellor. Carroll said he didn't have a definitive timetable on Thomas' recovery and did not know whether the safety would have to have surgery.
Thomas later told ESPN's Ed Werder in a text message that he is not planning to have surgery but intends to have the leg immobilized in a cast to allow natural healing, if possible.
Thomas also told Werder he still considers retiring from the NFL a possibility.
"I'm taking it one day at a time. I still feel the same way I felt last night," Thomas wrote in a text that he said could be made public.
During Sunday's game -- a 40-7 Seattle victory -- Thomas tweeted after the injury that the thought of retirement had crossed his mind.
This game has been so good to me no regrets.. A lot is running through my mind including retirement thanks for all the prayers.
— Earl Thomas (@Earl_Thomas) December 5, 2016
"He was going through at the time the emotional part of dealing with an injury that's a serious setback," Carroll said.
In another message to Werder, Thomas said, "You have to remember my team is still fighting I don't want to become a distraction."
A five-time Pro Bowler, Thomas had started every game of his career -- 118 straight games, playoffs included -- until he missed the Seahawks' Week 12 game with a hamstring injury.
Third-year player Steven Terrell will replace Thomas in the starting lineup.
Meanwhile, on the offensive side of the ball, a source told ESPN's Adam Schefter that the team intends to sign veteran fullback Marcel Reece.
Seahawks fullback Will Tukuafu suffered a concussion Sunday night against the Panthers.
Reece, 31, spent seven seasons with the Oakland Raiders and made the Pro Bowl three times. He appeared in 15 games in 2015.
The Raiders cut Reece in September after he served a four-game suspension for violating the NFL's policy on performance-enhancing substances.