We'll learn (much) more about the extent of this injury in the near future, though the Tribune has initially characterized it as more precautionary than anything else: "[Prior] broke off the mound on a ground ball to first and the Cubs training staff and pitching coach Larry Rothschild decided to play it safe."
Let's hope "play it safe" is the extent of it.
Update 9:30 PM ET: While there still is no official word, the Brewers broadcasters are saying Prior left due to some "tenderness" in his right elbow. There clearly isn't a whole lot of information available yet.
Update 11:50 PM ET:Confirmation that it was his right elbow that was tender.