Ronaldo produced a wondrous free-kick on the hour to send United eight points clear at the Barclays Premier League summit after Roque Santa Cruz had scored for Rovers, ending United's barren run at 1,334 minutes.
• Big Sam and Fergie agree to disagree
But visiting boss Sam Allardyce was left cursing referee Howard Webb for his failure to award a penalty when Rafael Da Silva appeared to foul Morten Gamst Pedersen as Rovers remained in the relegation zone.
Having gone so long without conceding, it must have been pretty galling United should lose their record in such a self-inflicted manner.
First, Nani and Paul Scholes waited for each other to collect a loose ball on halfway, allowing Andre Ooijer to step in and dart forward.
After beating Patrice Evra, Ooijer then slipped a pass to Santa Cruz. With Rio Ferdinand closing in on the South American, there was little need for Tomasz Kuszczak, standing in for the rested Edwin van der Sar, to dart out.
The situation still seemed to be under control. But both United players then committed themselves to a tackle, allowing Santa Cruz to calmly continue his run and tap into an empty net.
It was a run that had to end at some point of course. And at least Ferguson had the consolation of knowing his side had previously gone in front through Wayne Rooney.
The England striker had scored with his first touch following his return from a hamstring injury against Fulham on Wednesday.
Handed a start in order to prove his fitness ahead of the meeting with Jose Mourinho's Inter Milan, Rooney was buzzing about in his usual manner. And when Ryan Nelsen stretched to reach Nani's cross and succeeded only in prodding the ball slightly nearer his own goal, Rooney gleefully finished.
Nelsen had already saved a certain goal when he hooked the ball away from Michael Carrick's head as the England man arrived at the far post in the expectation of finishing off Ronaldo's cross.
Still, the hosts thought they had regained the initiative at the end of the opening period when Jonny Evans - selected ahead of Nemanja Vidic, like Van der Sar an ever-present during the previous 14 games but banned for the San Siro trip - rose to power home Nani's corner.
Referee Howard Webb detected a push, either from the Northern Ireland international or Ronaldo just in front as a group of players jumped. It seemed a very harsh call at the time and TV replays only backed up that opinion.
If Nelsen had seen his shot fly in instead of bouncing back off the post, Ferguson would have been complaining bitterly.
Instead, with El-Hadji Diouf firing the rebound straight at Kuszczak, United were soon celebrating.
The Red Devils have been waiting for a spectacular Ronaldo free-kick for about the same amount of time as their goal has been boarded up. Having lost out at one end, Ronaldo did the business at the other.
Paul Robinson must have been expecting a cross after Keith Andrews had sent Patrice Evra flying on the left edge of the box. Instead, the former England goalkeeper was beaten by raw pace as the ball flashed into the roof of his net.
Having lost one lead, it seemed unlikely the hosts would do it again.
Yet Rafael Da Silva was incredibly lucky to escape when he put his hands on Morten Gamst Pedersen's shoulders in an attempt to get into a tackling position as the Norwegian burst into the box.
Pedersen went down. Webb said no. On the touchline, Allardyce erupted in a rage and got involved in a heated exchange with Ferguson, with whom he had shared a convivial post-match drink at the same ground on Wednesday.
The departure of Evans straight after Ronaldo's blinding effort was a major concern, with Vidic suspended for the Inter clash.
Carlos Tevez's introduction at the same time was a reminder of the strength within Ferguson's squad, even if the Argentina forward is starting to mull over his long-term future.
Yet nowhere else, with the possible exception of Barcelona, would Tevez get to figure amongst such a dazzling array of attacking talent.
Fergie and Big Sam agree to disagre
Ronaldo, Dimitar Berbatov and Rooney, on three occasions, tried to increase United's account - but in the end it did not matter.
Old pals Sir Alex Ferguson and Sam Allardyce got involved in a furious exchange
of views as Manchester United scraped past Blackburn at Old Trafford tonight.But Blackburn were aggrieved Ronaldo was still on the pitch after escaping a
booking for a petulant kick at David Dunn.As he went on to pick up a yellow card for a dive, the Rovers camp knew the
world player of the year might have been sent off.And irritation turned to fury later on as Rafael Da Silva put his hands on
Morten Gamst Pedersen's shoulders inside the area, only for referee Howard Webb
to rule there had been no foul as the Norwegian hit the ground.''The boy Pedersen dived,'' argued Ferguson.''There is no question about that at all. Yet Cristiano Ronaldo gets booked for
the same thing and Pedersen doesn't.''Both cases were the same. You have to give either a penalty or a yellow
card.''Ferguson and Allardyce, who shared a post-match drink on Wednesday night,
clearly reached a different conclusion.''You would have to ask Howard Webb about the kick on Dunn. It was 56
minutes,'' said Allardyce.''As far as Pedersen goes, Sir Alex has his opinion, I have a different one.''Unfortunately, most clubs might not get a decision like that here. It wasn't
a blatant one but it was a penalty. Pedersen got pulled and tugged.''''It was a really bad goal to give away,'' said Ferguson.''It was so soft and indicative of our performance really.''Ferguson was left bemoaning a lengthening injury list ahead of Tuesday's
Champions League showdown with Inter Milan.Nemanja Vidic is suspended and both Gary Neville and Wes Brown have been ruled
out already. Now Ferguson seems certain to be without Jonny Evans, who limped
off immediately after Ronaldo's matchwinner.It means Ferguson is now crossing his fingers that John O'Shea recovers from a
sore heel in time to play against Jose Mourinho's side at the San Siro.''It is a nightmare,'' admitted the United chief.''Somewhere I have to find two centre-backs but it is looking very doubtful
because Jonny is unlikely to play now.''If O'Shea does not make it, Ferguson is facing the uncomfortable prospect of
calling up reserve team rookie James Chester or playing one of his more
experienced players out of position.Ferguson's concerns are very different to those of Allardyce, whose team visit
Coventry in an FA Cup fifth round replay on Tuesday still firmly entrenched in
the relegation zone.''We had a couple of opportunities to get ourselves in front, one just before
half-time and another after it when Ryan Nelsen hit a post,'' said the Rovers
chief.''It would have been a real bonus for us today.''
Manchester United 2 - 1 Blackburn Rovers
