Well, I guess I should have seen this coming.
Donald Thomas was placed on Injured Reserve on Tuesday, ending his season. The move was foreseeable once the team announced the signing of G Evan Mathis but did not announce that it had released anyone to make room on the roster for him. The room was made by placing Thomas on IR.
It's a bit strange that the team opted to place him on IR even though he was optimistic that he'd be able to play again. Apparently, he either didn't know the extent of his injury or the team was not willing to use up a roster space on injured player who wouldn't be able to contribute for an unspecified amount of time.
An injury like this is especially crushing for a team like the Dolphins because this season is all about rebuilding and getting young players ready to contribute in the near future. Promising rookies like Donald Thomas are the central components to that plan, and so it is particularly crushing to lose him for the season after just one game.
Ikechuku Ndukwe has been seeing the starting reps in Thomas' place, but I'd expect any of Shawn Murphy, Andy Alleman, or newcomer Evan Mathis to challenge him for that spot before long.