In a pulsating affair, second-half goals from Paul Scholes, Michael Carrick, Ji-sung Park and substitute Ole Gunnar Solskjaer cancelled out Blackburn's 29th-minute opener to strengthen United's lead at the top of the Premiership.
Watched by a record Premiership crowd of 76,098, the final score flattered United somewhat after a frustrating first-half display that saw the Reds lose Nemanja Vidic with a broken collarbone and go into the change-rooms a goal down.
Sir Alex made three changes to the United team that beat Middlesbrough in the FA Cup 12 days ago. Kieran Richardson made way for Ji-sung Park, while Paul Scholes returned from a three-match suspension to partner Michael Carrick in the centre of midfield. In goal, Edwin van der Sar started his first match since picking up a calf injury three weeks ago and needed to be alert early as David Dunn flashed a shot narrowly over the bar.
The Reds soon settled but found it difficult to create clear chances against a Blackburn defence well marshalled by captain Ryan Nelson. Ronaldo shot just wide of Brad Friedel's right-hand post inside 10 minutes before Wes Brown and Michael Carrick combined to mop up in the Reds' area.
The first real chance of the match fell to Wayne Rooney on 15 minutes after Ronaldo's long-range shot proved too hot for Friedel to handle. But the big American stopper recovered well to tip the England striker's follow-up around the post.