Rashford scored in the second-half of extra-time as the Red Devils sealed a 2-1 victory on Thursday and beat their Belgian opponents 3-2 on aggregate.
Henrikh Mkhitaryan had given United an early lead at Old Trafford but the hosts were pegged back when Sofiane Hanni equalised to send the game into extra-time.
The 19-year-old told BT Sport after the game: “All you can do is keep trying to create chances and that is what we kept doing.
“We pinned them back in the second half and started pushing as a team, which is what the game needed at that time.
“We had a lot of chances and could have scored a lot of goals but sometimes games go like that. We got one in the end and everyone is happy now.
“You just have to keep going and hope you get another chance. We have done it the hard way but none of that matters now.”
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