The Mexican striker came from the bench at half-time to score twice and create a third as the Red Devils overcame a two-goal deficit to seal victory.
And Sir Alex cited a summer break as the main reason for the 24-year-old's recent run of good form, before admitting he will struggle to leave him out of the first team.
|11/2||Manchester United are 11/2 to win the FA Cup with Paddy Power|
"When he came to us two years ago, that's what we were seeing more of today.
"And obviously, when you score a hat-trick you pick yourself, and I think he'll be playing next week."
The Scot also listed the attributes which he believes make the striker so effective, saying: "He does his natural thing - he's fantastic in the penalty box, his movement, his quickness and being in space; it's got him three goals today."