The 27-year-old defender penned a new deal with the Red Devils on Friday to keep him at the club until the summer of 2021, and with the option to extend for a further year.
The Argentina international has found his playing time to be limited this season due to a string of injury problems but Mourinho clearly values him as an important player in his squad.
The defender has only made seven Premier League appearances this season but will be hoping to prove his fitness in the closing months of the campaign.
And Mourinho has expressed his happiness at seeing the defender commit his future to the Red Devils by signing a new deal.
Mourinho told the Manchester United website: “Marcos has gone from strength to strength over the past couple of seasons, and has always shown great professionalism.
“Although his injury has kept him sidelined over these past few weeks, Marcos is working hard to return to full fitness.
“He is always ready to put his body on the line for this club and I am delighted he has signed a new contract.”
Manchester United are currently in second place in the Premier League table as they bid to try and finish in the top four this season under Mourinho.
The Red Devils were surprisingly dumped out of the Champions League last week after they suffered a shock 2-1 loss to Sevilla at Old Trafford in the last 16.
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