The Spaniard entered the match against the Danes at the start of extra-time, and made a near-immediate impact by creating two chances for Anthony Martial within four minutes.
The second of those chances drew the foul which earned United a decisive penalty, converted, as coolly as ever, by Bruno Fernandes.
Mata admitted the squad were relieved after a tough 120 minutes against stubborn and enterprising opposition, but felt the manner of victory underlined the club's determination to win the competition for a second time.
“I would say that today is more relief, to be honest, and tiredness!