The Premier League champions were two goals down heading into tonight's second leg at Old Trafford, but a Robin van Persie hat-trick secured their place in the quarter-finals.
"It is always tough not to concede a goal and score three but we managed it. There was pressure especially after [the loss to Liverpool on] Sunday, so we owed the fans a result like this," BBC Sport quotes the midfielder as saying.
"They were unbelievable. We have had good results and performances this season but have not followed it up. We won't get carried away now but we have to carry on like we did today."
United will learn who they will play in the last eight when the draw for quarter-finals takes place on Friday.