The Spaniard started as a substitute during Giggs's debut as United's stand-in manager but came off the bench to score twice after Wayne Rooney had put last season's title winners two goals in front.
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"That was a tough one, as it was leaving so many players out," Giggs told reporters.
"[Mata] was disappointed [not to start] 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."