The French striker won the decisive penalty as United booked their place in the semi-finals after Bruno Fernandes' penalty, and Hargreaves was full of praise for Martial.
Speaking on BT Sport: "Everyone's got an opinion on Martial. He's a Ferrari. He just looked like a £100 million player."
Martial's efforts were also lauded by United legend Paul Scholes, who described the 24-year-old as "unstoppable".
Scholes added: "Now I think he's brought the all-round game. He's got everything, he can beat a man, he can score goals, he's got everything you'd want in a centre-forward."
United manager Ole Gunnar Solskjaer is pleased with how Martial has developed through the season and described his display against the Danish side as "sharp".
The Norwegian coach said: "He was sharp.
"Anthony has developed throughout the season, he was involved with the penalty.
"He's impossible to stop at times when he gets the ball into his feet and he drives at people, his balance, he can go either way.
"The only thing now is to put that ball in the back of the net again. I'm delighted with his performance, though."
Martial is now United's top scorer this season as they look to end the season in European glory. They will face the winner of Wolves against Sevilla, who play on Tuesday (11.08.20).