If there's been one constant throughout Jose Mourinho's managerial career in England, it's been his love of bold substitutions.
Mourinho, perhaps more than any other manager in the modern game, uses his replacements swiftly, decisively and often unusually. He has made substitutions within the first 25 minutes to completely change formation; he has introduced players primarily tasked with communicating tactical messages to teammates; he's made a half-time treble substitution; he even once made a change in the dying minutes to introduce a tall player capable of marking the opposition goalkeeper, who Mourinho suspected would go forward for a late corner.
Mourinho's relationship with Juan Mata at Manchester United has been defined by two of these bold substitutions. Mata is not a typical Mourinho player -- that was evidenced by their uneasy period together at Chelsea, with the Spaniard struggling to adjust to Mourinho's demands for a counter-attacking wide player, despite having previously been Chelsea's Player of the Year in each of his first two campaigns at Stamford Bridge.
Mata left for United, so Mourinho's appointment at Old Trafford in 2016 was inevitably considered disastrous for the midfielder, who was immediately linked with a transfer away.
Indeed, in Mourinho's first game in charge of United, things appeared to be following the expected script: He brought Mata on as a substitute and then took him off again after just 27 minutes in the Community Shield victory over Leicester.
Mourinho cited tactical concerns, and a need for an extra aerial weapon to defend long balls, but a substitute being substituted remains the ultimate indignity in football. Mata, widely regarded as one of football's nice guys, a player who had never previously been seen losing his temper, was clearly unhappy.
An imminent exit seemed probable but, over two years later, Mata remains at Old Trafford. Now his role under Mourinho appears more important than ever.
Strange as it might seem now, it's only a month since it was reported that Manchester United would dismiss Mourinho regardless of the result in the home game with Newcastle. In the end, Mourinho stayed as United came from behind to beat Newcastle 3-2, but it was another of the manager's bold substitutions which proved crucial.
United played disastrously in the opening 10 minutes, going 2-0 behind. After just 19 minutes, Mourinho sacrificed centre-back Eric Bailly, shifted midfielder Scott McTominay into defence, and introduced Mata. In Mourinho's hour of desperation, two goals down to a relegation-threatened side and the axe supposedly looming, he turned to a playmaker he'd rarely trusted.
But Mata was the game changer. He dropped into central midfield to probe from deep, then shuttled forward into positions between the lines to create, switching play to the flanks. For United's first goal, his softly struck left-footed free kick was unusual for being both technically brilliant and entirely unspectacular -- clipped over the wall and into the net with a minimum of fuss. That's Mata all over: nothing flashy; simple quiet efficiency.
In the end United produced a positive, determined, relentless attacking performance to win 3-2 with three goals in the final 20 minutes; Alexis Sanchez finishing the job in the final few. It was the old Manchester United, the United of Ferguson: if they start the comeback, they'll finish it.
Something similar happened during the midweek win at Juventus, albeit with Mata introduced at a much later stage: 11 minutes remaining and United 1-0 down after Cristiano Ronaldo's thunderous volley.
Mata's influence on United's passing game was minimal in that short period, but again his set-piece ability proved transformative. United won a free kick in a very similar situation to his goal in the Newcastle game: the other side of the box, but very close to it. Sure enough, the execution was almost identical: so efficiently dispatched that goalkeeper Wojciech Szczesny got nowhere near it, despite the shot not being directed into the corner.
Mourinho appears to be coming around to Mata and it's easy to see why.
Mata has very rarely started in major games at all under the Portuguese (only five from 25 matches against fellow "Big Six" opponents before that Chelsea game), and seldom in the No. 10 position. But the intriguing aspect of the midfielder's deployment in recent weeks is the fact that he's started two matches against big opposition, away at Chelsea and in the first meeting with Juventus, in his favoured No. 10 role.
He wasn't handed that role to drift between the lines and create either ... he was tasked with a man-marking role on two dangerous deep-lying playmakers, Jorginho and Miralem Pjanic. For a player previously not trusted because of his defensive sloppiness, it can be considered quite a U-turn.
Mourinho may need someone to perform that role against this weekend against Manchester City. Fernandinho might not be considered a true playmaker like Jorginho and Pjanic, and Pep Guardiola's side possess many more creative talents. But so many of City's passing moves originate from the Brazilian, who is considerably less effective when shut down quickly.
Others may be more suited to that role, but Mata's recent combination of defensive diligence and key attacking contributions mean he's becoming increasingly difficult to leave out.
Mourinho and Mata remain a somewhat unnatural fit, and it's highly unlikely that both will be together at Old Trafford next season, with Mourinho never venturing into the fourth year at any club previously, and Mata's contract expiring.
But now, improbably, it feels like Mourinho is the more likely survivor. Mourinho has been detrimental to Mata's career, but Mata might just have saved Mourinho's career.