Juan Mata arrived at Manchester United as the answer to a problem, but now, to some, he is the problem that has been solved.
He was going to save David Moyes when he moved to Old Trafford in January 2014, halfway through that disastrous first season after Sir Alex Ferguson's departure. Four years later, Alexis Sanchez was going to save Mourinho from having to play Mata on the right.
In fairness to Mourinho, he does not see it like that. The week Sanchez joined from Arsenal, Mata signed a one-year contract extension.
"Mata is important," said United's manager. "He is another one, big story when I arrived one and a half years ago: 'Mata in trouble, in trouble, in trouble.'
"Now he's getting an extension of one more year. An important player for me, an important player for the club, an important player for the other players."
The "Mata in trouble" line was a reference to Mourinho's decision to sell the midfielder, twice Chelsea's player of the year, to United and Moyes. It followed that when Mourinho took over from Moyes' successor, Louis van Gaal, in 2016, he would look to do the same again.
Sources have told ESPN FC that Mata privately admitted concerns when it become clear Mourinho would get the job, and it took a conversation with the Portuguese coach during the summer to put him at ease.
Both Mata and Mourinho have maintained that their relationship off the pitch has always been good, even during those six months at Chelsea. But even if you discount their time together at Stamford Bridge, Mourinho's relationship with Mata the player has, at times, seemed odd.
This season is a good example.
Romelu Lukaku is the only United attacker to play more minutes than Mata in the Premier League. But they have played six games against the top six this season, and Mata has not started one.
Last season, only Zlatan Ibrahimovic and Marcus Rashford spent more time on the pitch than Mata. Yet he started just three of 10 league games against Manchester City, Chelsea, Liverpool, Arsenal and Tottenham.
Two were against Arsenal and Tottenham in May after Mourinho had already decided to focus on the Europa League. In the one other game he was picked to start -- a 1-1 draw against Arsenal in November -- he scored.
Mata has started important games under Mourinho -- the Europa League final for one -- but the manager's instinct has been to drop the Spaniard for key clashes, particularly in the Premier League.
He is, though, still here.
The early indications are that Mourinho has bought Sanchez to play on the left. It means Rashford and Anthony Martial, who have shared the job, will move over to the right, the position where Mata has played most of his games in the past two seasons.
Against Tottenham at Wembley -- another top-six game Mata missed out on -- Mourinho started with Sanchez on the left and Martial on the other side.
Mata, now 29, will tell you he is happy to play anywhere, but he still thinks of himself as a No. 10. It is not a role Mourinho has been keen to play him in -- at United or Chelsea -- and for now Jesse Lingard is filling that position, anyway.
If Lukaku and Sanchez are guaranteed starters, it leaves Rashford, Martial and Mata battling for one place on the right.
It will be a tough fight. But while Rashford and Martial are quick and direct, Mata possesses more finesse with his movement and his passing. In a big squad full of good players, it is no bad thing to be able to do something different.
The extra pressure on his place has not affected Mata's form. He has started 12 of United's past 15 games, only missing the Carabao Cup tie with Bristol City when he was rested and, predictably, Premier League games against Manchester City and Tottenham.
On Saturday against Huddersfield, it was his cross from the left that was turned in at the near post by Lukaku. The same combination gave United a 1-0 win over Bournemouth in December.
Mata has endured at Old Trafford. Having celebrated his four-year anniversary at the club last month, only seven current players have been in United's first team longer.
He has survived Mourinho's arrival, almost £150 million spent on attacking players like Martial, Angel Di Maria, Memphis Depay and Henrikh Mkhitaryan, as well as the emergence of Rashford and Lingard. He will hope to survive Sanchez, too.