The dismissal of Liverpool midfielder Javier Mascherano a minute before the interval of this clash between two bitter rivals at Old Trafford showed the enormity of the task ahead.
The Argentinian, already booked for a scything, 11th minute foul on United's Paul Scholes became embroiled in an incident which had nothing to do with him.
Referee Steve Bennett had booked Liverpool striker Fernando Torres for dissent and Mascherano, already walking a tightrope, fell off the high wire as he kept on talking to the official about the decisions he had made in the first half.
Mr Bennett showed Mascherano the red card and the controversy then spilled over with Liverpool boss Rafa Benitez and United's assistant manager Carlos Queiroz seemingly embroiled in a heated confrontation as the midfielder reluctantly left the field.
United were already one up at that stage through Wes Brown's header and went on to add late goals through Cristiano Ronaldo and Nani as they secured a deserved victory.
Had Mascherano stayed on it might have been different, but Untied are now hitting top form at the right time as the Premier League reaches its critical finale.
After a cagey opening, United were unlucky not to take a 24th-minute lead.
Ryan Giggs floated a free-kick from the right into the Liverpool box. The ball came off the head of Torres to Ronaldo who was lurking at the far post and he hit the foot of the woodwork with a close-range shot.
However, United had only ten minutes to wait before they broke the deadlock. Scholes' pass to the left was over-hit, but the magnificent Wayne Rooney refused to give up the chase for the ball.
His persistence paid off and his deep cross into the centre saw Brown out-jump Liverpool defender Martin Skrtel and goalkeeper Jose Manuel Reina to head home from six yards.
United probed Liverpool at the start of the second half but had to wait until the last 11 minutes before adding a second goal.
In the 78th minute, Carlos Tevez chested down Brown's cross from the right into the path of Ronaldo, whose shot was superbly touched behind by Reina.
But Liverpool's respite was brief as, from the resulting Nani corner, Ronaldo rose above Xabi Alonso to head home from six yards - his 34th goal of the season.
United's third - and best goal - came two minutes later. Scholes fed Rooney whose intelligent reverse pass set up Nani to score with a sweet right-foot shot into the bottom right corner of the net from the edge of the box.