The setback delays Alvarez's season debut, initially thought to be this weekend against the Yankees, and there's no way of knowing when that time will come.
"He played catch today, felt OK, but no timetable for the next time he'll be on a mound, either in a bullpen or in a game," A's manager Bob Melvin said Wednesday.
That the MRI didn't reveal anything alarming, Melvin said, "does give you some optimism."
Jane Lee has covered the A's for MLB.com since 2010. This story was not subject to the approval of Major League Baseball or its clubs.