The Spaniard has been constantly linked with a move away from Stamford Bridge in recent months—with Paris Saint-Germain among those most interested—having hardly featured in comparison to his involvement under previous regimes.
However, Duncan has shed new light on the situation, with Mourinho handing his playmaker increasing time on the pitch as the campaign wears on.
Speaking at Friday's press conference ahead of FA Cup third-round weekend, Mourinho commented on Mata's future at Stamford Bridge, denying any chances of a move away from the club (per Duncan):
I keep saying I want him to stay. Today he trained in a fantastic way, so I think he focussed more in what I told him after the game at Southampton, when I said I don’t want him to leave, he was focussed on that.
He was not thinking about when I said the door is open, he focussed on me wanting him to stay. He moved on, as I was saying after the game he was happy with the victory. Today he trained fantastically, there are no problems.
According to Transfermarkt.co.uk, Mata has started in just 11 of his side's 20 Premier League matches so far this season, and is yet to score a goal in league competition.
However, it is worth mentioning that the 25-year-old failed to play a full 90 minutes in the Blues' first 10 league outings, but has since played the entire match on two occasions, finding most of his starts after October.
WhoScored shows that, since 2011, there aren't many who have been more productive than Chelsea's out-of-favour star:
The Telegraph's Jeremy Wilson wrote just this week that Mata was interesting the likes of Manchester United, Napoli and Inter during winter, but added on Friday that it was PSG who are most likely to stump up the kind of fee necessary to prise the maestro away from west London.
Mourinho's return to Chelsea hasn't run quite as smoothly as some might have proposed leading up to his return, but the club still find themselves third in the Premier League table and through to the knockout stages of the Champions League.
The former Real Madrid boss has stumbled upon some of the player-manager-relationship obstacles that sometimes blotted his time at the Bernabeu. Sergio Ramos was one one of those reported to have not always gotten along famously with his boss, per Goal.com's Ben Hayward.
As Chelsea enter the second leg of their campaign, it will be crucial that the English giants make full use of every exhaustible resource at their disposal, and Mata should yet have plenty left in the tank.