Here is the latest I could find:
Sheets Leaves Game Early
Sheets Out After Five Innings
Michael Ryan
Milwaukee Brewers pitcher Ben Sheets left the game early on Friday after feeling tightness and stiffness in his right tricep in their game against the Cincinnati Reds at Great American Ball Park.
Sheets pitched five innings, giving up only two hits and no runs. He walked none and struck out four after after firing only 60 pitches. He did not come out to start the sixth inning.
Sheets lowered his ERA to a microscopic 0.96. He is scheduled to make his next start Wednesday night at home against the Philadelphia Phillies.
Doesn't sound that serious unless its some kind of tear...with Sheets I wouldn't doubt it.