Starter motor/flywheel on Ford Transit

I recently had my starter motor replaced on my ford transit approximately 6 weeks later it is given me trouble. My mechanic said it was probably a faulty part, I asked could it be my fly wheel. He replied that he had checked it before replacing the motor and it was in perfect condition. When he checked it this time 10 teeth were missing. Could that be as a result of the starter motor being replaced wrongly? Thanks in advance. Niall