The Athletics activated Clarke (hamstring) from the 60-day injured list Tuesday.
Clarke has been out of commission since late April, first due to a bone bruise in his foot and later because of a severe hamstring strain. The outfielder has hit only .214/.262/.323 with a 38.8 percent strikeout rate in his first 219 big-league plate appearances. However, Clarke's elite defense could mean regular reps in center field down the stretch, with Henry Bolte shifting to left field.