Mata poked home a Phil Jones cross shortly after coming on to replace Danny Welbeck on the hour, and nodded in a deflected Antonio Valencia effort as well as tormenting the visitors with some of his trademark subtle touches as Giggs’ side sought an even greater winning margin.
Saturday’s match was the first time the Spain international had started on the bench for United since his move from Chelsea in January, with the only other times he has sat it out being the Champions League ties for which he was cup-tied.
But the Reds’ interim manager admitted that this decision, along with the other calls he had to make on the players he has worked alongside for so long, was one of the toughest he had made since stepping in as boss for the remainder of the season.
“That was a tough one, as it was leaving so many players out,” said the United legend.
“He was disappointed, and I'd expect that. It would be strange if he wasn't, but he just showed the class that he's got both as a player and as a professional. He put on a half-hour masterclass, he was brilliant and I was really happy for him because it's not easy because he's got a big price tag.
“To not start the game was disappointing for him and I'm really pleased that he came on and made such an impact.”