Thomas was injured in the first series of Sunday's 42-10 loss at St. Louis, but was able to play the rest of the way.
The linebacker underwent an MRI on Monday and the diagnosis was a sprained medial collateral ligament. Thomas also reportedly has a small tear in the ligament.
The Dolphins were off on Tuesday, and it wouldn't be a surprise if Thomas sat out practice when the team returns to work on Wednesday.
One thing is certain: Thomas wants to be careful with the injury. He learned his lesson last year when he sustained an ankle injury, missed a game, came back, only to aggravate the injury.
Thomas then was forced to sit out four games and was never 100 percent the rest of the way.
"Last year, if I would have just sat out one more game I wouldn't have missed three more," Thomas said. "I would have been playing the rest of the year. But I was impatient. That was my decision. You've just got to be patient.
"Yeah, it's an important game and a division game, but if you're not ready, then you miss more. I'm going to be patient with it and see how I feel out on the practice field."
Thomas has also revealed that he tore some cartilage in his right knee during training camp.
Coach Dave Wannstedt has said Tommy Hendricks is next in line at middle linebacker if Thomas is sidelined.
Hendricks has basically played nothing but special teams since joining the Dolphins last year.
In some Tuesday news, the Dolphins made a practice squad transaction, waiving running back Derek Combs and signing linebacker Kevin Rollins.