1) Used tweezers to pick broken plastic fragments out of the hole that contains the metal shaft that mates with the plastic sprocket.
2) Dropped the new plastic sprocket onto the metal shaft, then fit the turntable over it. Unfortunately, the plastic sprocket's shaft was too long, preventing the turntable from resting fully on the floor of the microwave.
3) Used some rough sandpaper to shave a few millimeters from the plastic sprocket's shaft.
4) Redropped the new plastic sprocket onto the metal shaft, then fit the turntable over it. This time, the table rested properly. All done.