#2 Juan Mata instead of Javier Hernández (Summer 2011)

It's easy to argue Javier Hernández's tenure at United was a success; a good goal scoring record, cult hero status and a two premier league winner's medals. "Chicharito" was a favourite among the Old Trafford faithful.
However, this was an era where United were crying out for creative players. Hernández wasn't that, he was a goal poacher and a "super-sub" waiting to come on a nick a goal. Useful in many situations, but his contribution wasn't enough to truly make his mark.
In late 2011, Wayne Rooney threatened to leave United due to a lack of talent coming in. Hard to argue when Rooney had seen United's rivals in Chelsea and Manchester City bringing in world-class talent in the summer of 2011. Most notably, Juan Mata arrived in England that summer.
United eventually got their man, but far too late. Juan Mata was one of the stars of the Valencia team pushing Real Madrid and Barcelona every season in Spain and other regulars in the UEFA Champions League.
David Villa and David Silva had already been snapped up by Barcelona and Manchester City and it seemed the third world cup winning Spaniard was soon to follow in leaving Valencia, and it was Chelsea that acted first.
Mata's time at Chelsea earned him a UEFA Champions League Winners medal and two consecutive player of the year awards, and the biggest thing he offered Chelsea's team was creativity; something Manchester United were crying out for in the early 2010s. United should have acted earlier and bought Mata three seasons before they finally did.