Coach Chuck Pagano announced on Thursday that Thomas suffered a torn right quadriceps and would be lost for the season, "We got an MRI [Wednesday] night and he did re-injure that quad," Pagano said, via the Colts official website. "We feel awful for Donald. He was playing extremely well."
Thomas will join running back Vick Ballard on injured reserve. Ballard was lost for the 2014 season on July 25 after tearing his Achilles in practice.
As for Thomas, the injury marks the second time in 11 months that he's torn his right quad. The 28-year-old missed the final 14 games of the 2013 season after tearing the quad in Week 2 against the Dolphins.
The 2013 season was Thomas' first-year with Colts and he started both games he played in last season. Thomas was projected to be the team's starting left guard this year, but now that job will likely fall to rookie Jack Mewhort, a second-round pick out of Ohio State.