Sir Alex Ferguson has repeatedly claimed this summer that United will prosper without the world's best player, sold to Real Madrid for a world record £80million.
For that to happen, Rooney must become United's main man.
And, after playing second fiddle for so long, and spending too much time on the wing for his liking, Rooney returned to centre stage in every sense, rising to meet Nani's cross, then tapping home when his 34th-minute header bounced back off a post.
The sense of change around Old Trafford was made even more obvious by the absence of Rio Ferdinand with a thigh injury, meaning a first Premier League start for Fabio and the captain's armband for John O'Shea.
It seemed a strange decision by Sir Alex Ferguson, who in the past has extolled Rooney's leadership virtues.
Maybe the United boss felt on such a big day for the England man, it was unnecessary to give him the added burden of leading his team. And, in fairness, as skipper or not, Rooney was at the centre of his side's most inventive moments.
Ronaldo's absence will be felt for some time. There was none of that magical trickery the Portugal superstar has provided for the Old Trafford faithful for six stellar years and none of that edge-of-your-seat anticipation either.
Yet in terms of chances, the hosts created plenty, fuelling a belief that they will be a more rounded team.
Nani in particular looks to have been injected with a fresh dose of confidence. The winger was not quite as effective as he was during last week's Community Shield encounter with Chelsea but his direct running caused Birmingham problems, as did Antonio Valencia on his debut as he drifted in from the other flank.
If the Portugal international had got his shooting boots on, United would have been in front before Rooney struck.
As it was Nani scooped a disappointing shot over from Patrice Evra's cut-back and set the stage for Rooney to make his mark.
The striker was involved in the build-up, as he linked up with Dimitar Berbatov to release Nani down the left.
Rooney climbed above Franck Queudrue to reach the teasing cross and although his header bounced back off the post, he was perfectly positioned to tap home the rebound.
Birmingham's response was spirited and after Cameron Jerome's shot had flicked off Paul Scholes, Gregory Vignal thought he had equalised from the corner, only for Evra to nod his header off the line.
It was a sporadic flurry of attacking play from the visitors, who were thankful not to be further behind at the break given Evra set up Fletcher with a clear chance in stoppage time, only for the new Scotland skipper to steer his shot wide when it looked easier to score.
Having come so close in the final seconds of one half, United nearly doubled their lead in the opening moments of the next.
When the ball bounced invitingly 30 yards out, Rooney could not resist the temptation to let fly, drawing another good save out of Hart.
United's first decent post-Ronaldo free-kick was taken by Ryan Giggs after Vignal had caught Rooney late on the edge of the area.
The veteran's effort would not have been far off either if it had not flicked wide off Jerome, who had worked tirelessly as a lone striker at the other end without threatening.
A similar statement could not be made about his replacement Christian Benitez, who found himself streaking down the left with 13 minutes to go and, after stepping inside Wes Brown, with a clear sight of goal.
Some pundits wonder why Fabio Capello keeps faith in Ben Foster given the number of times he has been unavailable for England duty. In a split-second he provided the answer.
As Benitez opened out his body out and went for the far corner, Foster flung himself in the same direction, flicking the ball away to let United breathe again.
Substitute Michael Owen did have the ball in the net once, only to be flagged offside. He was most certainly on when he raced onto Rooney's chested through ball in stoppage time
Manchester United utility man John O'Shea has backed Michael Owen to score plenty of goals for the Premier League Champions.
