Alex Ferguson's side won 1-0 at Old Trafford and so they will be Watford's opponents in the last four on the weekend of 15th/16th April.
In the other quarter final replay Chelsea beat Tottenham 2-1 at White Hart Lane and they will now face Blackburn Rovers in the other semi final.
In the other quarter final replay Chelsea beat Tottenham 2-1 at White Hart Lane and they will now face Blackburn Rovers in the other semi final.