Manchester United have gone more than a month without bringing in a first-team player - and here is the reason.

Supporters' concern over the club's lack of transfer movement this summer is understandable. This is not normally how they go about their business.

Michael Carrick's side have brought in Andrey Santos and Youri Tielemans to strengthen the midfield, along with Karl Darlow as a back-up goalkeeper - and that is all so far.

Yes, 18-year-old Cristian Orozco has also arrived from Colombia, along with two academy additions, but overall it has been a rather unusual summer for Manchester United.

Are the club short of cash? Is Sir Jim Ratcliffe now pulling the purse strings tighter right at the top, rather than INEOS's cost-cutting measures falling disproportionately on ordinary staff?

The explanation is simpler than that: Manchester United have spent heavily in recent times, and even one of the richest clubs in the world has to accept that some tightening is needed when the return on investment has been poor.

In simple terms, Manchester United are not broke, nor are they skint - they are an organisation simply too big to fail, in the same bracket as Real Madrid or Barcelona. But there comes a point when transfer spending judged to be poor business starts to have consequences.

That is the situation Manchester United find themselves in this summer. Last year's spending on Matheus Cunha, Bryan Mbeumo, Benjamin Sesko and Senne Lammens was substantial, even if the club did succeed in moving Alejandro Garnacho and Antony off the books.

Poor Michael Carrick, then? Not really - the tiny violins can be put away, because the club have still spent a hefty £85 million on Santos and Tielemans.

According to English football finance figures from Valuball, Manchester United's net spend on player trading since the start of the 2022/23 season stands at £614m. Unsurprisingly, only Chelsea have a higher figure.

Unlike the Blues, however, who have recouped almost £1 billion through player sales over the same period, Manchester United have developed a reputation for being poor sellers. Across five years, the club have generated only £280m in sales, which is less than Leeds United, Bournemouth and Nottingham Forest.

Over time, an inability to move on expensive deadwood - or even to sell that deadwood at a decent price - starts to restrict incoming transfer activity.

From this season onwards, the existing Profit and Sustainability Rules (PSR) are being replaced by the new Squad Cost Ratio (SCR) model. Under these rules, a club's total on-pitch spending, including transfer amortisation, player wages and agent fees, is capped at 85 per cent of its overall football-related revenue.

That means the profit a club makes from player sales directly shapes how much it can spend.

Even so, Manchester United's revenue is far greater than that of most of their Premier League rivals, so there remains every chance they will make more signings in this window and not feel the pressure as sharply as a club without such enormous commercial income.

Clubs without Manchester United's off-field resources simply have to become smart sellers to stay afloat and remain competitive in the Premier League - that is the reality of the situation. Chelsea, in particular, have become very shrewd in this area, which has so far helped finance their huge spending since the Roman Abramovich era ended.

Manchester is once again a two-team Champions League city this season, and that is a major financial boost. But if those in charge cannot consistently make profits on players over the long term, then silence in the transfer market could soon become a familiar feature of every summer at Old Trafford.