The best time to buy is from a position of strength. It serves various purposes, suggesting to winners that standards will have to be met, or raised, and acting as a form of intimidation to ambitious rivals. It is a statement of intent that Sir Alex Ferguson has made several times before.
Having won his first Premier League title, he broke the British transfer record to recruit Roy Keane. After his seventh, he did so again when buying Juan Sebastian Veron, as well as adding Ruud van Nistelrooy to the ranks. When the league was reclaimed in 2007, Carlos Tevez was signed, and when the Champions League was regained 12 months later, Dimitar Berbatov arrived.
By the same rationale, the ideal occasion to part company with players is when on top. That, too, delivers a message, by dispensing with the deadwood and looking to bring in quality. So while it might seem an odd assertion to make about the probable champions and possible Double winners, here it is: Manchester United need a clearout. This should be the final campaign at Old Trafford for a sizeable portion of the squad.
There are several factors. Theirs has been a strange season of mediocrity and magnificence, where ageing players and age-old habits of indomitable spirit, improbable comebacks and invaluable late goals can serve to mask failings. A 19th title would be a remarkable achievement but a final points total in the low 80s would show the scope for improvement.
United are a club who have benefited from continuity, but it can be taken too far. The high-profile departures of Cristiano Ronaldo and Tevez have camouflaged the reality that, in the last two-and-a-half years, only one other regular in the squad, Ben Foster, has been moved on. The assumption of permanence seems to have led others into a comfort zone. Though he was bizarrely awarded a new contract recently, Michael Carrick is an obvious example: he no longer passes the ball forwards with invention, incision or penetration.
Wes Brown and Jonny Evans are others to have relapsed and, though there should still be time for Evans to rediscover his form, the former is 31, lacking confidence and conviction and enduring a third successive undistinguished season.
All three are on the wrong side of the unofficial divide in Ferguson's squad. It is sizeable, but uneven: the 30-or-so players can be split into a first 15 who have made considerable contributions and a second group who impress too infrequently. Often as Carrick is involved, he belongs in the latter category.
While some have deteriorated, others have never excelled to begin with. Bebe and Gabriel Obertan are cases in point, their efforts in the Carling Cup defeat at West Ham and the FA Cup win over Crawley sufficient to show they are not United players. Nor is an oft-forgotten man, Mame Biram Diouf, who has been loaned to Blackburn but is increasingly unable to get into their team. With Danny Welbeck and Tom Cleverley due back at Old Trafford next season after loan spells elsewhere, Ferguson will have superior options on the flanks without spending a penny.
Others will leave of their own volition. Gary Neville spared his manager a difficult decision by retiring, albeit eight months after he should have done. Edwin van der Sar is resistant to entreaties to play on for another year and, sadly for admirers of his talent, Paul Scholes has been reluctant to sign a new contract as well.
Should he exit, the need for a midfield makeover would be obvious. As it is and for very different reasons, United supporters could be forgiven for echoing Giovanni Trapattoni's suggestion that Darron Gibson looks for first-team football elsewhere. The Irishman has not progressed beyond enthusiastic shooting, meaning he lacks the all-round game to play as one of two central midfielders.
In contrast, Owen Hargreaves possesses every attribute but fitness. There have been rumours of a pay-as-you-play deal when his four-year contract expires in the summer, although, as it stands, it would be the least lucrative agreement in footballing history. It means that, should Scholes retreat into retirement and semi-hibernation in Saddleworth, only two of United's six specialist central midfielders - Darren Fletcher and Anderson - deserve to be at the club next season.
Nor are there grounds to retain United's other Owen - Michael - who has been rendered redundant by Javier Hernandez, currently providing the razor-sharp finishing and impact from the bench that the Englishman was supposed to. Owen is an old 31 and evidence of his decline was apparent long before United signed him, while Sampdoria loanee Federico Macheda is another due to return to Manchester next season. Following the January signing of Anders Lindegaard, meanwhile, goalkeeper Tomasz Kuszczak is another set to lose his place in the pecking order.
Collectively, there are reasons for a dozen departures. If that is unlikely, the only basis to keep some is if Ferguson lacks the funds to replace them or if buyers do not materialise, although the stubbornness that seems an essential part of any great manager's make-up may prevent him from discarding some of his more ill-advised buys. In much he does, Ferguson is a contradiction. He is a man who can veer from loyalty to brutality rapidly but, rather than celebrating a momentous season, he should show his ungrateful side. There is a case to trim his herd.