United defeated Copenhagen 1-0 to reach the Europa League semifinals.
And Hargreaves has heaped praise on Martial, who has enjoyed arguably his finest season at United with 23 goals in all competitions.
He told BT Sport: "Everyone's got an opinion on Martial. He's a Ferrari. He just looked like a £100million player."
And Hargreaves wasn't the only one impressed, with Paul Scholes adding: "He's turning into a No 9 more and more every game I watch.
"He's normally a brilliant finisher but tonight the finishing wasn't quite there."
Another former United hero, Robin van Persie, was also in awe of Martial's performance and highlighted the Frenchman's importance under Solskjaer.
The Dutchman added: "Martial always have an eye for his team-mates and that's what sets him apart from the others.
"That's why I think he's a world class player."