David Moyes took a huge gamble on the England international's talents by giving him a lucrative new contract and the striker must repay his coach's faith.
Still today, he is undoubtedly the key man in David Moyes' new-look United; the man the Scot has appointed his Chosen One, and on whom rests his chances of success in the job.
Ahead of Tuesday's key derby against City, question marks still remain as to whether Moyes' gamble on such a hugely talented yet occasionally volatile individual will pay off, or prove to be the beginning of his downfall.
In the hope of convincing the Old Trafford faithful of the former, and with Nemanja Vidic heading to Inter, the England international is expected to be handed the United captaincy from the beginning of next season.
It is a remarkable turnaround. When Moyes first drove into the champions' Carrington training ground, Rooney was widely seen as an outcast, with Sir Alex Ferguson having claimed in the immediate aftermath of his final game in charge that the 28-year-old had again handed in a transfer request - an accusation the player denies to this day.
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Rooney and the Scot are now closer than ever, after the former committed his future to the club. Indeed, there is a feeling that Moyes craves familiarity. He spent 27.5 million pounds to acquire his former Everton enforcer Marouane Fellaini in the summer and made his first job as United boss convincing old pal Rooney to stay.
Yet the latter charm offensive has seen other players grow distant. Robin van Persie, notably, has privately expressed his frustrations at Moyes' intense training regime, fearing that he has once again become susceptible to the niggles that plagued his time at Arsenal. Rooney, tellingly, has spoken of his love of the workouts.
Van Persie has aired other grievances in public, complaining that many of his teammates were repeatedly occupying the "zones" in which he preferred to play. It was seen as a thinly veiled attack on Moyes and, despite his later apology, he is believed to be unsettled at Old Trafford and may be more so after his injury fears were realized last week.
The Dutchman was stretchered off in the dying stages of the club's 3-0 Champions League win over Olympiakos, having already scored a hat trick. If he is indeed out until the end of the season – as has been suggested – then those three goals may be the last he ever scores in a United shirt.
It is a quite remarkable role-reversal, with Van Persie suddenly on the fringes and Rooney a key ally of the manager. When United celebrated last season's title triumph Sir Alex warmly embraced Van Persie on the podium, as he sent a clear message that the £24 million signing had as good as won the title by himself. Rooney was afforded a curt handshake.
That was just minutes before Sir Alex dropped his transfer bombshell on live television. The fans turned on Rooney. The majority of the squad was on Sir Alex's side – believing the striker was merely trying his luck once again. The first time he asked for a transfer he was rewarded with a £250,000-a-week contract.
Had Ferguson had his way Rooney would have been gone long before the Community Shield clash with Wigan Athletic. But he instead deferred to Moyes and the junior Scot made a conscious decision – without Sir Alex's guidance – to keep the striker.
He topped up his contract by £50,000, making him the highest earner in English football history, while Vidic is simply keeping his captain's armband warm.
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It is a risky strategy from Moyes. He is so intent on keeping Rooney happy that he has, arguably, limited other star players. Juan Mata, the club's record signing from Chelsea, has become a left winger, with Rooney often deployed in the Spaniard's preferred position of No.10.
Moyes appears ready to bow to his wishes – irrespective of the team's subsequent shape or formation – but it has not helped so far. The fact that United is seventh in the league, 18 points behind Chelsea and without any hope of qualifying for the Champions League via the top four, would appear to suggest that Moyes would be better off prioritizing team harmony over one individual's happiness.
Yet Moyes has bet it all on red, despite doubts from a number of senior officials at the club. It could well prove to be a master stroke, but Rooney is an explosive individual both on and off the pitch.
If he has his head turned again by the glitz and glamour of City, or the riches of Paris Saint-Germain, then there will be little to stop him finally leaving United. After all the club can hardly offer him any more money.
The roulette wheel, however, is already spinning. Moyes has handed over his stake and must pray that the ball lands in the right slot. If it doesn't, he will be out of a job. Risky, indeed.