Jose sold him to us for £37 million because of his preference for a pacy, powerful, physical Chelsea team and for Mata - who is massively popular both at the Bridge and in Manchester - his days seemed numbered.
He was voted Chelsea's player of the year in successive seasons but fell out of favour under the Portuguese because of an apparent failure to work hard defensively and stick to his role wide on the right.
The Spaniard scored his third of the season in last night's win over City and the stats - although admittedly they can be misleading - highlight his growing influence on this side.
One of the few players to have emerged from the last few turbulent years with an enhanced reputation, Mata has scored 30 times, assisted 18 and is injury free, which is always a precious commodity.
What's more, most of those have turned out to be crucial, match winning moments: think of his goal in the cup final, last night and his 'Juanfield' double in 2015 to name but three.
In his nine starts this season, we've won seven and lost only once (at Feyenoord) and without him in the team, we've lost three, drawn one and won at Northampton in the cup.
If that doesn't tell you of his importance to the team I don't know what will.
Mata is often criticised because he does not fit the "United identikit": pace, counter attacking and flying wingers but he's the archetypal number ten and I can't think of any that possess speed.
His vision and ability to find space make up for it, though, and despite his diminutive stature, he is not afraid to dig in and take a game by the scruff of the neck.
He's pulling the strings in attack tucked in behind Zlatan and is the one player who looks the most likely to carve open defences.
Mourinho still has decisions to make as to who plays the number ten role but it is becoming increasingly hard to ignore the gifted and mercurial Spaniard.
He glides through games with such ease and is crucial to the way we want to play when expected to dominate the ball.
Not only is Mata a wonderful and highly intelligent player, he has also earned a reputation as one of the nicest blokes in football.
Before the Stoke game earlier in the season he stopped the team coach to take the time to pose for photographs and sign the shirt of a young, disabled fan.
Not for nothing has Juan Mata become my favourite United player (he evens ends his blog posts with 'hugs')