Inter frustrated as Man Utd lock up midfield target
Mata's deal was due to expire at end of the season, but the Red Devils have opted to activate their option to prolong his stay by a further 12 months.
And United boss Jose Mourinho, who sold Mata to United from Chelsea in January 2014, talked up the attacking midfielder's importance to his plans.
"Mata is very important. When I arrived one-and-a-half years ago, [they said], 'Mata is in trouble, in trouble, in trouble', and now he's getting an extension of one more year," Mourinho told a news conference ahead of Wednesday's trip to Wembley to face Tottenham.
"An important player for me; important player for the club; important player for the other players."
The arrival of Alexis Sanchez at Old Trafford from Arsenal has led to rumours that Mata could return to former club Valencia, or join Inter Milan.
But Mata spoke of his desire to remain a United player. "I still feel very happy and very proud to be part of this club," he told MUTV. "I think everyone that belongs to Manchester United should feel privileged, and that's how I feel.”