"I feel sorry for the players that I change, but I am not here to be nice to the players, I am here to think about the team," was his response to BBC Sport on the back of Chelsea's 3-0 win over Southampton on New Year's Day.
It was all sparked by Mata's reaction to being substituted after just 53 minutes, being replaced by Oscar, who himself went on to make headlines for a whole host of other reasons.
Up to now, Mata has remained dignified throughout what has been a frustrating campaign for him personally.
From being Chelsea's back-to-back Player of the Season, he has been forced into cameo roles for the Blues this term, struggling to make an impact under Mourinho.
He has done it all largely with a smile on his face, though, with little complaining, hiding any disappointment from public view.
Yet it all came to a head at St. Mary's when the Spaniard showed his displeasure at being hauled off shortly after the interval, cutting a frustrated figure in the dugout.
Since then Mourinho has hinted Mata can leave the club if he so chooses.
"I want to keep him. I don't want him to go. That is my opinion, my wish, but my door is open," reported ITV Sport.
"The club's door is open, too, so when a player wants to speak to us we are there waiting for them. If you are asking do you want the club to sell him? I don't want to."
Anyone predicting this situation six months ago would have been made a laughing stock. Mata was simply too vital for his club to even consider allowing him to leave, but here he is, seemingly surplus.
How can a player once so valuable now be seen as dispensable at Chelsea?
3) Rafa Benitez deepens Chelsea fans' love for him by making a move to buy Juan Mata for Napoli http://t.co/bnRyk3Zzdt— Mirror Football (@MirrorFootball) January 2, 2014
It's a question many fans want answered and one Mata himself must be asking as it's still unclear. What is certain, though, is the impact Mourinho's treatment has had on Chelsea's No. 10.
Without regular football, Mata is a shadow of the player English football has adored since he moved to Stamford Bridge from Valencia in 2011.
An essential part of Chelsea's success in the 2011-12 Champions League and last season's Europa League victories, Mata's name among the protagonists of Chelsea's recent good form has been replaced by Willian, Oscar and Eden Hazard.
With just one goal to show for his efforts—coming in the Capital One Cup against Arsenal—it really has been a season to forget.
Being played out of position on either flank, he cannot impact games the way he has done in his previous two seasons in West London.
Andre Villas-Boas, Roberto Di Matteo and Rafa Benitez all played Mata through the middle. He was the man they relied on, and he rarely let them down.
The figures speak for themselves—13 goals and 21 assists in his debut campaign were followed up by an even more impressive 20 goals and 28 assists last year.
Goals alone, that's a return to rival Frank Lampard at his peak. Add the assists, and it outlines the player Chelsea have.
There may be defensive shortcomings on Mata's part, but tactical work on the training ground can overcome that aspect. And besides, at a time when Chelsea continue to struggle for consistency in front of goal, an individual of Mata's ilk would inject some much-needed killer instinct.
Should Chelsea look to sell Juan Mata?
As well as victory over Southampton, the Blues welcomed 2014 with news of a £49.4 million loss for the year ending June 2013. It's a significant figure and one that, in these times of Financial Fair Play, suggests Mourinho may be forced to sell before he can add reinforcements to his squad.
Mata cannot be one of those who departs, however. Especially when there's plenty of deadwood elsewhere at Stamford Bridge. The likes of Kevin De Bruyne are far from Mata's quality, bringing little in return for the investment the club has made in them.
Just as time is rightly being afforded Mourinho in his quest to make Chelsea the Premier League's top team once more, it needs to be given to Mata also.
In the long term, it's Chelsea who stand to lose the most.
Garry Hayes is Bleacher Report's lead Chelsea correspondent and will be following the club from a London base throughout the 2013-14 season. Follow him on Twitter @garryhayes