Yet again United's success was based upon keeping a clean sheet, the fifth successive match they have prevented the opposition from scoring and their fourth 1-0 win.
But it was not an easy success against a battling Birmingham City side who produced their best performance of the season but gained no reward for their endeavours.
It is now 20 games since Birmingham last beat Manchester United and this was possibly the closest they came to ending the dismal run when they were punished for their one mistake of the match.
They made a very smart start against an apparently casual United and could have gone ahead in the opening couple of minutes.
Paul Scholes was caught in possession and the ball ran loose into the path of Cameron Jerome who saw his powerful shot parried around the post by Edwin Van der Sar. From the corner Van der Sar was again extremely cool, plucking Franck Queudrue's header out of the air.
In some fast open play Wes Brown found Birmingham's defence out of position and Maik Taylor was forced to rush from his line to make a save as Wayne Rooney attempted to lob the keeper.
The surprising feature of the game was that Birmingham were able to pressurise United's formidable defence. Seb Larsson cut through and provided a cross which Gary McSheffrey headed goalwards only to see Rio Ferdinand clear off the line.
United's sheer class was always evident in their swiftly executed moves and Birmingham were given a warning of what could be in store when Ronaldo shot on the turn straight at Taylor.
Carlos Tevez was not so prominent and he could make little impression against on-loan Johan Djourou and Liam Ridgewell, who is proving to be a bargain buy for Birmingham.
Rooney, too, was not as sharp as usual and this explained why United were unable to open their account in the first half.
But five minutes into the second half the less than popular Ronaldo put his stamp on the game when he capitalised upon a mistake by Queudrue and made the Birmingham defender pay dearly for his transgression by calmly racing forwards, before slotting his low shot past Taylor.
It put the champions on course for another victory despite a fighting Birmingham display which deserved at least a point.
United, however, were unable to coast home as Birmingham continued to be a major threat as McSheffrey illustrated when his deflected shot off Ferdinand was brilliantly saved by substitute Tomasz Kuszczk, who replaced the injured Van der Sar for the second half.