I think that's probably precisely the reason they'd go with Moyes in the first place. Anything is going to be a step down from the greatest club manager in the history of the game. If they bring in Mourinho and it all goes to buggery without Red Nose at the helm, the trout storm would be colossal. So they can just bring in some random bum like David Moyes under the provision of "look how well he did at this club despite having nothing" and then if it all goes well and he manages to cope at United, lovely, everything is rosy. If the post-Taggart United fall to pieces, "Well, we took a punt..." and now it's time to bring in the big guns when the expectations are lower and the manure fans are over the whole "WHY ISN'T EVERYTHING AS GREAT AS IT WAS WHEN SIR ALEX WAS HERE URRRR WE'RE THE BIGGEST CLUB IN THE WORLD WE HAVE HISTORY" thing.
