His hat-trick was also his 12th in the competition, surpassing the 11 of Alan Shearer.
Hargreaves, a former team-mate of Aguero at City, says Aguero’s third goal at Villa Park epitomised how difficult he is for defenders to stop.
“The one thing you used to realise about Sergio is when you got the ball in to him, in and around the box, the way he just shifted the ball with his low centre of gravity, you couldn't get anywhere near him,” Hargreaves said.
“By the time he got it out of his feet and shot, the ball was already in the net.
“Defenders hadn't even really adjusted to the first move, that's how sharp he was.
“And we saw that, I think, on the last goal. It just happened so quickly, and he gets it out of his feet, and he shimmies and he shoots almost all at the same time.”
Hargreaves says that City’s style of play under Pep Guardiola allows Aguero to thrive, with more records in the Argentinian’s sights.
"In tight spaces, with these guys, with David Silva, [Kevin] De Bruyne and [Riyad] Mahrez, the service he gets, he's just in the perfect team,” Hargreaves said.
“That's probably why he's stayed nine years, because the service that he gets allows him to get a lot of these goals, and the way Pep plays suits him perfectly.”