Dimitar Berbatov celebrates his second goal with Wayne Rooney
Manchester United strolled to victory to leave Celtic's Champions League hopes hanging by a slender thread.
Dimitar Berbatov looked offside when he flicked home a close-range finish to put United ahead after 30 minutes.
And there were similar doubts about his second, turned in on the rebound after Celtic keeper Artur Boruc could not hold Cristiano Ronaldo's free-kick.
Wayne Rooney emphasised the gulf in class between the sides with a 20-yard shot on the turn with 14 minutes left.
United are now sitting pretty at the top of Group E and on course to qualify for the knockout stages, but Villarreal's win against Aalborg leaves Celtic with an uphill task to emerge from the group phase.
Berbatov form pleases Ferguson
Gordon Strachan's side was below-strength and it showed as they never looked like making a dent in a dismal Champions League record on their travels that now reads played 17, lost 16 and drawn one.
But they will also point to two debatable United goals, although in reality they were massively inferior to their English counterparts.
Manchester United lost Rio Ferdinand with a groin injury, but Ronaldo kept his place despite speculation that he might be rested - and he played his part in a vibrant first-half display from Sir Alex Ferguson's side.
It was Celtic, however, who posted the first danger after 11 minutes when Aiden McGeady forced Edwin van der Sar into a save low to his left with a 20-yard shot.
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