Well technically they'd be thinking "I thought he'd retired... why the **** has he come to manage us?"
Fergie hasn't retired, he's just waiting for a bigger club to swoop for him. Maybe City after Guardiola flops.
Biggest question mark over Moyes was his decision to join Man United, not the actual job he did. Really calls into question his judgement. You could say once in a lifetime opportunity, but his stock was VERY high, I would question the wisdom of anyone who took that job after Fergie. Literally an impossible job.