Out went David Robertson and Craig Conway, who dropped to the bench and cup-tied duo Danny Grainger and James O'Brien.
in the 18th minute the Rangers captain Barry Ferguson forced Dundee United's goalkeeper, Łukasz Załuska to produce the first real save of the game.
Before the second half got underway, Rangers manager Walter Smith made a substitution with midfielder Brahim Hemdani making way for Striker Jean-Claude Darcheville.
United felt they should have been awarded a penalty in the 53rd minute when Cuellar appeared to pull back Christian Kalvenes in the box, but referee Kenny Clark inexplicably waved play on.
The first half of extra time got underway with Rangers looking the better team, however with just 5 minutes played Mark de Vries restored United's lead when he put the ball past McGregor with a strike from the top-right edge of the box.