In the top of the second inning, Justin Masterson left the game after a second injury timeout. Later in the game it was revealed that he left because of an injury:
Indians announce that Masterson exited with soreness on left side of rib cage.— Jordan Bastian (@MLBastian) September 2, 2013
That usually equates to some sort of strained abdominal muscle, usually an oblique muscle. If that's the case, there's a good chance that Masterson's season could be over. Needless to say, losing Masterson for the remainder of the season would be a huge blow to a club that had already seen its playoff chances dim after a 1-5 week.
The good news is that Corey Kluber will return this weekend, and the Indians will also have Carlos Carrasco and Josh Tomlin with the big league club starting tomorrow. But Kluber is going to have to work himself back into starting shape, and you don't really want to count on Tomlin or Carrasco to carry you down the stretch.
Let's hope that this was all a precautionary move, with Masterson being available to make his next start.