Colts coach Chuck Pagano announced the extent of Thomas' injury following Sunday's 24-20 loss to the Miami Dolphins.
Thomas, who was injured in the first half and had to be carted off the field, took to Twitter following the game to thank his supporters.
I truly appreciate all the prayers and support today from my fam teammates and fans! My body will heal and I will b back! #coltsnation
— Donald Thomas (@Thedthomas64) September 15, 2013
Rookie Hugh Thornton, the Colts' third-round pick draft pick, replaced Thomas and will be the starter going forward.
"You hate to lose anybody," Pagano said. "[We] feel awful about Donald. That's how the game is. I thought Hugh came in and did an admirable job of filling in for him."
Thomas is the second Colt to go down with a season-ending injury in less than a week. Starting running back Vick Ballard suffered a season-ending knee injury in practice this past week.