"He's getting an MRI right now," A's manager Bob Melvin said after a 6-2 loss to Detroit on Wednesday night. "So, we won't know anything until he gets back."
Anderson fell forward and landed awkwardly after throwing his 48th pitch with the bases loaded.
Oakland trainer Nick Paparesta and manager Bob Melvin talked with Anderson briefly on the field before deciding to take him out.
"We could see earlier in the inning that he was trying to stretch out his back, so I don't think the fall had anything to do with it," Melvin said. "We just checked him at that point and decided we didn't want to go any farther with him."
Melvin plans to pitch Dan Straily in Anderson's spot in the rotation if he can't make his next start.
Right-hander Pat Neshek replaced Anderson with one out in the third inning. Delmon Young, the first batter Neshek faced, hit a sacrifice fly to give the Tigers a 3-0 lead and then Jhonny Peralta grounded out to end the inning.