Thomas, who has missed practice time but no game time with chronic turf toe, said he hurt his right hamstring in the first quarter of the Bears' 27-24 win over Tampa Bay.
Bears coach Dick Jauron, who reported no major injuries on Monday, said Wednesday that Thomas hurt the hamstring "a couple of weeks ago."
Thomas is the NFC's leading rookie rusher with 638 yards on 150 carries, and trails just San Diego's LaDainian Tomlinson, who has a league-leading 794 yards on 217 carries among rookies.
"It's sore but it feels better than(Tuesday)," Thomas said. "It hampered me more Sunday than I thought it would. Running backs are going to get hurt, it comes with the position."
Thomas, who ranks fifth in the NFC in rushing and 12th overall in the league, missed two preseason games with nagging injuries and has been slowed by his injured toe the past month. The quiet rookie from Michigan,however, didn't hesitate on his game status for the Vikings at the Metrodome.
"I'll be there -- most definitely," Thomas said. "I've played through injuries before."
Jauron wouldn't say whether Thomas would get the start, but said missing practice could cost him. Thomas will most likely sit out today's scheduled practice and could be in jeopardy of starting if he doesn't get any reps in by Friday.
"He's been bothered by a couple of things the last couple of weeks, but he's not a guy that complains," Jauron said. "It's obviously bothering him. We could (sit him), but we definitely don't want to do that. On the other hand, you don't want any player out there who's injured."
Veteran James Allen, who lost the starting job to Thomas in Week 6, will get the nod in the backfield if Thomas isn't set to go.
"I'll be ready, but we'll just have to wait and see how it goes," said Allen, who is averaging just 3.1 yards a carry compared to Thomas' 4.3.
"I'm always penciled into the game plan, so we'll just have to see how it goes."
Since his demotion, Allen has played in mostly third-down passing situations. He ranks third on the team in receiving with 26 catches for 171 yards and a touchdown.
Thomas, who has averaged 102 yards per game over the past five games, had consecutive 100-yard games against Cincinnati and San Francisco. But he was limited to just 58 yards on 23 carries in the win over Tampa and had just 45 yards on 22 attempts against Green Bay the week prior.
In other news, Marty Booker was named Miller Lite Player of the Week following his career-high 165 yards (seven catches) and three touchdowns against Tampa Bay.
The Miller Lite Player of the Week honor marked only the third time in the 13-year history of the award that a Bears player has been recognized.