It was his first for the club and the first by any United player in this fixture since Stan Pearson in September 1946.
"I'm told it's the first hat-trick for more than 60 years for a United player against Liverpool," Sir Alex told MUTV.
"That tells you what a big achievement it is. We're very pleased for him because his form has been excellent.
"There was a lot of criticism of Dimitar in the media last season. It happens when we buy a player for a lot of money and they don't score a hat-trick in every game. That's the way of the world for some players who come here.
"This season he's started off in the right fashion. His pre-season training was very good, he was good on the pre-season tour and he's carried that on. But there's never been any doubt here about the quality of the man, never any doubt. We've seen his quality again today."