I'm just being practical here. Last minute goalkeeper transfers are problematic by themselves, and on top of that foreign goalkeepers always struggle to adapt to PL's aerial bombardment. I don't think United can afford signing a midget goalkeeper who may take half a year to adapt. Casilla can walk in and play his best on day one, because this is what he does. He's great shot stopper and a big, commanding figure at the back.
I can see where you're coming from on that one, but even the context of the league taken into account I still don't think he's as good. The aerial threat of the PL is somewhat overstated IMO.