I don't think it was his fault with the failure at Man Utd as Fergie left a very poor team, he knew what he was doing in his last season by signing van Persie as a quick fix to get him goals then to go out on a high so to speak.
But IMO Moyes has dropped a bollock by going to a struggling Spanish side, his tactic's just won't suit their type of football. He would have been better biding his time and getting the job at Villa, Newcastle or that type of club.