But as the game headed towards a penalty shootout, Greenwood emerged from a 10-strong crowd of substitutes to seal the victory that United deserved.
The 17-year-old showed great composure to turn inside his man and bury a shot in the Inter net with 14 minutes to go.
Only moments later he clipped a cross from fellow sub Tahith Chong on to the crossbar but he had already done enough to make it three wins out of three for Solskjaer's tourists.
United midfielder Andreas Pereira praised Greenwood at the final whistle.
“Hes scoring goals, we're very happy for him. Hopefully he can keep it up and score more goals," Pereira told MUTV.
“I know him from the youth. Every chance he scores."