The movable barrier operator 10 is a shaft-mounted movable barrier operator that operates a garage door 20. Such an operator includes a motor 22 operably connected to shaft 26 such that motor 22 may move door 20 between the open and closed position. Shaft 26 may include axle 36 and wheels 38. The movable barrier 10 further includes a rotational drive unit 46 with a sun gear 52 having an open center 49 into which the axle 36 extends. Further, the rotational drive unit may have an annularly shaped portion 47 and a removable portion 48 such that the operator 10 may be installed onto a previously mounted garage door. The movable barrier operator may also include protection plates or a finger guard 90.