Requiring only a point to lift their 12th championship of Sir Alex Ferguson's incredible reign, United had fallen behind in the first half of Saturday's Premier League clash at Ewood Park when Brett Emerton's far-post drive gave Blackburn a shock lead.
With Tomasz Kuszczak's nerves affecting his team-mates, for a while, United looked to be heading for a nerve-jangling final-day ending with Blackpool.
But help was at hand in the form of Rovers keeper Paul Robinson, who needlessly slid out on Javier Hernandez.
Much to Robinson's frustration, the Mexican went down and after referee Phil Dowd had restored order, Rooney drilled home to finally knock Liverpool off their perch, as Sir Alex Ferguson had promised all those years ago.
Travelling north in their thousands to herald a record title and put a damper on anything that might happen at Wembley a couple of hours' later, United being behind at half-time simply was not in the script.
The visitors had played with the confidence of a team who felt they were about to become champions, stroking the ball around efficiently and creating plenty of space around the Rovers box.
But that killer ball was missing.
After Nani had thudded a close-range header against the bar with only three minutes on the clock, United just could not find their range.
Nani cut a shot past the near-post, Ryan Giggs brought an excellent save out of Robinson and Wayne Rooney's chip sailed narrowly over.
United actually looked less likely to score as that opening period went on.
This would not have been too bad if Blackburn were scoreless too.
However, a combination of their perilous league position and being piqued at having their own position completely overlooked in the scramble to acclaim the men from Old Trafford, they approached their task with a fierce sense of determination.
Christopher Samba had already been slightly unlucky that Phil Dowd had ruled his excellent effort out for a foul on Nemanja Vidic when Emerton drifted a cross-shot to the far-post.
Kuszczak tried to push the ball away from danger but succeeded only in keeping it in play, which allowed Martin Olsson to retrieve it.
After exchanging passes with Junior Hoilett, Olsson crossed to the far post where Emerton finished with an excellent first-time effort.
It nearly got even worse for United when Michel Salgado's cross from the other side of the pitch dropped nicely for Olsson, whose shot was disappointingly off-target.
However, Ferguson must have been questioning the wisdom of his only concession to United's position, offering 40-year-old Edwin van der Sar a rare break, as Kuszczak had twice almost been caught in possession, having dwelt on routine back passes for far too long.
Quite apart from putting United's celebrations on hold, it also highlighted just what a tough job Ferguson faces in making sure the veteran Dutchman's replacement is a worthy successor.
Javier Hernandez has already proved his value to the Red Devils cause.
The Mexican was not having one of his better days though, failing to collect a Ryan Giggs' through ball that would have sent him clear of the Blackburn defence.
More weak goalkeeping from Kuszczak forced Rio Ferdinand into a rushed clearance before Olsson nodded an Emerton cross against the post with the United keeper nowhere.
Another Emerton cross led to Ferdinand getting rather too close to heading into his own net for the central defender's liking.
The hesitancy Kuszczak created had spread through the United defence.
Normally the strongest of partnerships, Vidic and Ferdinand crashed into each other as they tried to complete a routine interception on halfway.
Something needed to go United's way. Robinson's rashness provided it.
The former England keeper complained bitterly at the outcome but it is hard to imagine any striker not going down in such circumstances.
Blackburn were angry. Not angry enough to warrant caution being thrown to the wind in an attempt to win though.
Their top-flight survival fight will go down to the last day, when United will receive the Premier League trophy as the most successful team in English league history.