From the Tigers' official Twitter page:
Tigers have recalled Galarraga from Toledo today and he will start on Tuesday. The club optioned Oliver to Triple A Toledo today.
This move comes as no surprise for two reasons: 1) We already knew Galarraga was scheduled to start on Tuesday; and 2) Oliver was as good as gone yesterday after another bad pitching performance. He started off pretty well with the Tigers, but his last few starts have just been awful, so it was definitely time for a change in the rotation.