It was, in truth, a decision that had been coming for weeks now. Wayne Rooney's form did not justify his place in the starting XI, and so he was replaced.
Mourinho made several changes to his starting line-up against Leicester, and while some were enforced, others were significant.
The Portuguese manager has shown a willingness to experiment with his wide players so far, with the likes of Juan Mata, Marcus Rashford, Henrikh Mkhitaryan, Jesse Lingard, and Anthony Martial all given starts in those positions.
However, he has been unwilling to tamper with his central players. In every game bar the opening day (before Paul Pogba had completed his transfer), he has used a midfield pairing of Marouane Fellaini and Paul Pogba, with Rooney ahead of them in the number 10 role.
Against Leicester, however, that changed. Fellaini was replaced with Ander Herrera, while Mata was given a chance to play in his natural position instead of out wide. Rashford and Lingard started on the flanks.
And it worked. The team thrived, and two players who were already in the team benefited greatly from the changes. Those two players are Zlatan Ibrahimovic and Pogba.
Ibrahimovic was able to drop deeper for the ball and lead the attack to great effect, without worrying about occupying the same spaces as Rooney. He didn't score, but gave arguably his best all-round performance in a United shirt. His four goals so far have come about due to his sheer quality, but with Mata supplying him and 'fast kids' around him, we may begin to see him more involved in the play.
However, it wasn't just the link up with the forward players that impressed. Against the Foxes, we witnessed a growing relationship between Pogba and Ibrahimovic. Mata's movement in the free role vastly contrasted to Rooney's, often drifting across the pitch to influence the play and open up gaps rather than dropping deeper or staying central for long periods. This created space for Pogba to drive into and play his natural game. His goal came from a corner, admittedly, but his confidence was given a massive boost. He produced an incredible shot from distance, and played Ibrahimovic in more than once with a clever chipped through-ball.
Pogba's attacking performance would not have been possible, though, without the outstanding work of Herrera. Seemingly considered too positionally ill-disciplined by previous manager Louis van Gaal, he proved his former manager completely wrong here. He was committed to protecting his back four, sitting deep and making numerous important challenges. And when he got the ball, he had the quality to start attacks from deep.
This meant that while United lined up nominally in a 4-2-3-1, they actually played more like a 4-1-4-1 in possession.
The focus to this point has mainly been on the central players, but it is important to note the impact of United's wide men. Without their pace to stretch the game and create chances, much of the work of the central players would likely have been futile, suffocated by a congested midfield.
The difference is that Mourinho has plenty of options out wide. Rashford, Lingard, Mkhitaryan and Martial all have pace, trickery, and quality in the final third.
There were various other positives to take. Blind was excellent at left-back, for example, deputising for the injured Luke Shaw, and reminding the world that he made his name in that position, rather than at centre-back, despite his obvious affinity with Eric Bailly.
This result must not be taken out of proportion, though. It was an excellent result and a tonic for a team struggling with poor form. However, three of the four goals did come from set-pieces, and United must really push on from here if they are to make the most of a squad that is bursting with talent.
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