Maguire is a good stopper, but he has a playing style that suits a packed relegation team more than a supposed top team. Sure he has some patience on the ball and can actually pass it to a team-mate, but he is first and foremost and good warrior and header of the ball. Fearless when the ball is in the ball, wins lots of headers every game, great asset for a bottom team being bombarded for 90 minutes.
Top teams need smart defenders, like Rio who knows where the ball is going to go before the pass is made. Calm, in control, great tackling when needed. Maguire is slow and reactionary, has never played for a top backline and will struggle at United in my opinion. In the WC he looked like any other player when facing a top team and England let in goals.
I like him, he is a tough bastard, but he will not make us particularly better and be the kind of player that charges us towards the gold. Not even close, and feck that figure quoted for him.