United held a 3-0 lead going into the return, with Sir Alex Ferguson leaving Wayne Rooney on the bench and handing the midfield anchor role to Alan Smith.
The home side missed a good chance early on as a corner was nodded down to skipper Tamas Sandor, but he blazed over from eight yards out with his left foot.
United then lost Gary Neville with a groin injury that will keep him out for at least a month, before Smith went in the book with 14 minutes on the clock.
However, six minutes later Gabriel Heinze gave the visitors the lead, heading home unmarked at the far post from Paul Scholes' right-wing corner to extend the aggregate advantage to four goals.
Ronaldo tested Norbert Csernyanszki on the half-hour mark with a low shot, with United controlling the play with some ease up to the half-time whistle, before Edwin Van der Sar had to make a superb save to block Sandor's 20-yard free-kick.
Straight after the restart Van der Sar was forced into action by Tibor Dombi's fierce shot from a tight angle, but United soon regained their composure to prevent their hosts threatening the Dutchman's goal again.
Heinze doubled the lead on the hour, powering a header past Csernyanszki from Gigg's accurate free-kick from the left.
Kieran Richardson soon added a superb third goal, driving home a left-foot shot from 20 yards following good work from Ronaldo.
Ruud van Nistelrooy dragged a shot just wide of target 15 minutes from time, while Ronaldo almost got on the scoresheet after a superb late run.