Manchester United need defensive stability and purchase one of the most eccentric defenders in European football. This does not make sense to me when there are more natural defenders on the market. What worries me is his style of play, mentality and crucially his positioning at centreback but you may just get away with this with the new 3 at the back.
For me he's a disaster waiting to happen just like David Luiz, the Barclays Premier League is testing for the most experienced ball playing defenders let alone a young man of his calibre. I hope for you that this is not the case because he's an important part of the Argentinian project in the next 5-10 years and they need him to develop.