Will Carroll is a doctor whose hobby is analyzing sports injuries. He also writes for the Baseball Prospectus and runs his own blog here. In his Baseball Propectus column today, he writes the following ominous news from the Stros:

The Astros may have completed a two-game sweep of the Cubs, but losing Andy Pettitte for any length of time would certainly be a high price to pay for that sweep. Pettitte left after four innings with elbow soreness, later clarified to be “forearm tightness.” While Pettitte insisted after the game that the pain was in a different area than the elbow injury earlier this season, his reactions on the mound were very similar. A shot of the dugout showed him rubbing not his forearm, but his elbow. Team physicians will examine Pettitte Thursday morning.

Update: In this Chronicle article, the Stros attempt to put the best face on the initial diagnosis (strained forearm) as possible, but the reality is that Pettitte has a gimpy elbow and throwing pitches only aggravates the injury. Accordingly, I expect Pettitte to go back on the disabled list again at least once and maybe more during the remainder of this season.