The most successful manager in Premier League history left Old Trafford in 2013 after 26 years in charge.
His departure marked a downfall in United's dominance with David Moyes and then Louis van Gaal failing to achieve the expectations required despite huge money being spent.
Manchester United fans rage at starting XI to face Anderlecht Thu, April 13, 2017
Manchester United fans are not happy with Jose Mourinho's team selection ahead of their Europa League quarter-final first leg against Anderlecht (8.05pm)
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Manchester United fans fume at the Red Devils' starting XI against Anderlecht
On his arrival at Old Trafford, Mourinho also spent big to bring Paul Pogba, Henrikh Mkhitaryan, Eric Bailly and Zlatan Ibrahimovic to the club.
It has so far resulted in the EFL Cup and a place in the Europa League quarter finals but United face a battle to finish in the top four of the Premier League.
Manchester United boss Jose Mourinho
And Mourinho thinks money does not guarantee success and revealed he is tasked with modernising United.
He told ESPN Brasil: “It’s been a little forgotten to follow the evolution of the times at practically all levels. Then, when a mythical manager left, there was obviously a void, a void that fostered stagnation. The club stopped a little.
Sir Alex Ferguson won the Premier League on 13 occasions
“It is important to modernise the structure. Without a good structure, there are no good dynamics. Without good dynamics, there are no results. And people sometimes think that money buys players and players buy success. Not here.
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“Before buying players and before thinking about a high level team, you have to think about modernising the structures, giving new dynamics, because at this moment football is so professionalised, there are so many areas of support. If we do not keep up with evolution, it’s complicated.”