Things could not have been worse for United in a disastrous first half.
They lost key defender Nemanja Vidic in the 28th minute when he was carried off on a stretcher with a shoulder injury after landing badly when he went for a Ryan Giggs corner. He was replaced by John O'Shea.
Within a minute United were behind when Michael Carrick's attempt at cutting out a Morten Gamst Pedersen cross from the left saw his mis-timed interception wrong-foot goalkeeper Edwin Van der Sar.
The ball rebounded off the Dutchman's chest for Matt Derbyshire to hook his close-range effort into the roof of the net.
United were abysmal in that first period with Wayne Rooney's wretched international form for England repeated at club level.
He missed two golden opportunities, being denied by brilliant saves from Blackburn goalkeeper Brad Friedel on both occasions.
The United team which came out for the second half proved a totally different proposition from their pale first-half imitation.
Rooney should have scored soon after the restart when he planted a free header wide from close range and then shot tamely at Friedel from 12 yards.
But United were now stirring themselves from their first-half lethargy and, sparked by the imagination of Cristiano Ronaldo and the intelligence of Paul Scholes, they equalised in the 61st minute.
The goal was a Scholes gem. The former England man, returning after a three-match ban, picked up a loose ball just outside the Blackburn penalty area.
As United's Artful Dodger jinked between and then passed three Blackburn defenders, he worked the space to shoot and buried the ball in the bottom corner of the net with a crisp low right-foot shot from 12 yards.
United should have taken the lead in the 68th minute. Rooney completely mis-kicked with the goal at his mercy from four yards when he went for Ronaldo's low cross from the left by-line.
The ball fell to Giggs, who crashed his shot against the underside of the bar from eight yards.
The miss seemed to fire up United even more and they took the lead four minutes later.
Giggs worked a short corner with Ronaldo, who skipped passed Tugay on his way to the left by-line.
The Portuguese star's low cross went back across the penalty area through a ruck of players to Carrick on the far side of the box and he scored with a slide-rule right-footed shot from 12 yards.
Blackburn were now reeling before United's all-out attack. Christopher Samba handled the ball on the Blackburn D and Ronaldo stepped up to blast the resulting free-kick goalwards.
Friedel did well to parry the shot, only to see Ji-Sung Park follow up and score with a right-foot tap-in from six yards.
United completed the scoring seconds into stoppage time. Rooney's surging run forward saw him lose the ball on the edge of the box, but Park retrieved the situation to find Ole Gunnar Solskjaer free on the right side of the Rovers penalty area and he scored with a crisp low shot from eight yards.