The Red Devils desperately need centre backs, with Nemanja Vidic and Rio Ferdinand's departures leaving United somewhat short - particularly if Louis van Gaal presses ahead with his three-man back line. As a result, new manager van Gaal wanted to bring Belgian international Thomas Vermaelen from Arsenal to Old Trafford, with the 28-year-old's confidence in possession a huge factor.
However, the centre back is now on his way to La Liga giants Barcelona after a deal between Arsenal and United fell through today. Reports suggest that the reason for the collapse is because Arsenal demanded United defender Chris Smalling in exchange - a deal that van Gaal wasn't willing to sanction.
Given that the Dutchman was so against letting Smalling go, it raises the question - just how important will the former Fulham man be this season?
The 24-year-old is one of just three senior centre backs at Old Trafford, alongside Jonny Evans and Phil Jones, so letting one go to bring one in seems incredibly counter-productive - that will no doubt have been a factor in van Gaal's decision. Yet the biggest indication of Smalling's importance could be demonstrated in the Netherlands' World Cup side under van Gaal this summer.
At the World Cup, van Gaal used a back three as well, and one of the keys in the defence was Bruno Martins Indi. His ability to play both centre back and left back enabled him to shift over and allow wing back Daley Blind to push forward without leaving space in behind.
With a big bid in for Fiorentina's Juan Cuadrado, Smalling could play a similar role, only on the right hand side. Cuadrado's natural instinct is to attack, and with Smalling having experience at both centre back and right back, it could be the perfect match in van Gaal's system.