Coach Dick Jauron revealed the severity of Thomas' injury for the first time today, but declined to rule out the player for the rest of the season. Jauron added Thomas is not expected to require surgery. He was hurt in a 36-14 loss at Jacksonville last weekend.

Thomas was briefly spotted after practice in the trainer's room walking on crutches and favoring his right leg. The seven-year veteran has 89 yards rushing this season and has been predominantly used in third-down situations and as a backup to rookie starter Marshawn Lynch.

Lynch is still recovering from a sprained left ankle and expected to miss his third straight game this weekend. That leaves Buffalo (5-6) with two unproven running backs: Fred Jackson, who is likely to start, and rookie fourth-round draft pick Dwayne Wright.

Jackson, an undrafted free agent who spent his rookie season on the Bills' practice squad last year, has nine carries for 34 yards and eight catches for 55 yards in three games this season. Wright has 26 carries for 77 yards and three catches for 17 yards, while being active in all 11 games this year.

"It's a great opportunity for me," said Jackson, who had 18 yards rushing and added five catches for 47 yards against the Jaguars. "To go out and play against a team like Jacksonville and go out and do some of the things I was able to do ... it was a great building block for me, so I'm looking forward. This is the next step."

Jackson had spent the past few years playing in NFL Europa and the United Indoor Football League since completing his college career at Division III Coe College in 2002.

Starting defensive end Chris Kelsay (sprained left ankle) is also expected to miss Sunday's game. Kelsay, who was hurt against the Jaguars, still wore a protective boot as he watched practice from the sideline.