United's club record £37.1m man came off the bench to score twice in the rout of the Canaries having been omitted from the starting lineup by the Welshman on his managerial debut.
However, Giggs was quick to praise the Spaniard for the professionalism shown to shelve his disappointment over not making the first XI by proving his mettle on the field.
He 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."
Wayne Rooney also bagged a brace in the win, sending the Red Devils on their way with a first-half penalty before doubling the advantage three minutes after the interval.