The 19-year-old striker scored an extra-time winner as the Red Devils saw off the Belgians 2-1 in the second leg for a 3-2 aggregate triumph to advice to the semi-finals.
Rashford, who provided the assist for Henrikh Mkhitaryan's opener, has now scored three goals in his past four outings having found the back of the net just three times in six months prior.
"You can speak about Marcus's quality and he has that and that is fantastic but he was not scoring a goal since September," he said.
"But even when he didn't score [from September until April in the league] he kept confidence from me.
"He is fantastic in his effort."