We're often told about the importance of marginal gains not just in football, but in sports as whole. Cut ketchup out of the club canteen, get each player his own linens on away trips, hire an extra Ph.D. to crunch numbers -- that sort of thing. So it's sometimes a bit mysterious when clubs occasionally ignore the biggest "marginal gain" of all (actually, there's nothing marginal about it), which is having the players they want and need when preseason training begins.

We hear managers talk about the importance of signings being given time to settle, of team chemistry (both tactical and social) and of customized training camps. And then their clubs go and sign players at the very end of the transfer window, sometimes once the season has already begun. It's one of those rare situations where you can actually measure the outcome, at least to some degree.

Cast your mind back nearly 11 months to week four of the 2015-16 Premier League season. Manchester United travel away to Swansea and lose 2-1. Sergio Romero, playing in goal instead of David De Gea (who was left out by coach Louis Van Gaal because he's on the verge of a move to Real Madrid) makes an almighty blunder as Bafetimbi Gomis' shot squirms under his body. At the other end, there is no Antony Martial because, well, he has yet to sign for Manchester United.

There's no way of telling for sure, but it's not unreasonable to suggest that with De Gea and Martial on board, United don't lose that game. Or, at least, that there is a far greater probability they get a draw or maybe even a victory. It's by no means certain -- De Gea does makes mistakes, remember -- but if it's about fine margins, you'd rather have the Spaniard between the sticks.

What would an extra point have meant? Well, fourth place, for a start, which would have turned into an extra £1.2 million ($2m) in prize money. Oh, and a chance to play in the Champions League this year, which could have translated into an additional £25m ($33m) to £60m ($80m) depending on how far they get, plus box office receipts.

It also would have brought some peace of mind. Should things go badly wrong this season -- knock on Wood(ward) -- and United fail to finish in the top four for the second consecutive year, then the much-discussed £75m ($99m) a season deal with Adidas gets slashed by 30 percent, which means United lose another £22.5m ($29.7m). And maybe, with a fourth place finish, Van Gaal might even still be in a job. (OK, I realize that rather undermines my argument in favor of early transfer dealings, but bear with me.)

I'm not picking on United here. There are plenty of clubs who leave it very late. Some have no choice: they lose a starter and they have to scramble for a replacement. Others engage in a game of chicken, waiting until the last minute because they are haggling over money or because the guy they want will only come if the selling team gets a replacement, thereby creating a domino effect that sometimes isn't resolved until the very end.

Others still are forced to move late because the manager decides in training camp that he needs reinforcements or a player gets injured. But when it comes to the big boys, it's harder to understand why deals get dragged out and pushed to the last minute. And, like I said, it's not just United. Manchester City waited until Aug. 30 to sign Kevin De Bruyne for a whopping £62.5m ($82.6m) last season. Juventus only sealed the deal for Hernanes on August 31. Each transfer tells its own story, but at this level, it usually boils down to either a first-choice target getting away or haggling over money and contracts.

Hindsight is 20/20, of course, but in a game of fine margins, the impression is that many clubs seem to value the points at stake in August less than doing their business early, even if it might mean paying a premium. And they seem to value the "coachspeak" about the importance of training camps and having time to work with the players even less. (As most will tell you, there's really no time to do any meaningful coaching or tactical work during the season, simply because of fixture congestion.)

I have no idea if somebody at United figured out that if they had signed Martial earlier (and not gone into the season with Wayne Rooney and James Wilson as their sole striking options), or if they had resolved the De Gea mess earlier (so that Romero wouldn't be on the pitch), there would probably be Champions League football at Old Trafford. What I do know is that it's not coincidental how key business is being done early with Jose Mourinho at the helm.

Zlatan Ibrahimovic, Henrikh Mkhitaryan and Eric Bailly were all signed and sealed by mid-July. The Paul Pogba deal is dragging on, of course, but that's what can happen when you're dealing with a world record transfer, and in any case, it's still early August. Mourinho is reportedly looking to add a defender, too: odds are that if it happens, it will happen by the middle of the month.

Jose Mourinho has turned Man United around this summer and has done so with a flurry of early transfer business. Nils Petter Nilsson/Ombrello via Getty Images

Look back over the last decade or so of Mourinho's career and a pattern emerges: Big players get signed early. Other than Samuel Eto'o (which occurred on Aug. 29, 2013), Pedro (Aug. 20, 2015) and Wesley Sneijder (Aug. 27, 2009), virtually every player his teams have added arrived before the league season began, often far before. And, it should be added, each was something of a special case. Eto'o and Pedro came in not as automatic starters but as complementary players, while Mourinho had to persuade Inter to spring for Sneijder; as it happens, he was proved right because the club went on to win the Treble.

Mourinho obviously doesn't negotiate transfers himself, but he's the guy identifying targets early and then needling and harassing the club to get things done. From that perspective, he's a significant upgrade over Van Gaal: In two summer transfer windows alone, Van Gaal signed more starting calibre players (Martial, Daley Blind, Marcos Rojo and Angel Di Maria) after the domestic season had begun than Mourinho has in the past decade. (For the purposes of this discussion, we're not counting Papy Djilobodji, no offense).

It doesn't take a genius to figure out that when a team (especially a bigger and wealthier club) goes on a shopping spree just before the transfer window shuts, something isn't quite right.

Cast your mind back to the last 48 hours of the 2011 summer window, when Arsenal picked up Per Mertesacker, Mikel Arteta, Andre Santos and Park Chu Young. It's not that they were poor players -- well, it's fair to say Andre Santos and Park contributed less than expected -- but that they were thrown in at the deep end and it took time for them to find their feet. (It's no coincidence that Arsenal were 15th as late as October, though they'd eventually recover and finish third, a point clear of Tottenham -- sound familiar?)

The rules are somewhat different for clubs outside the very top tier. Sometimes they get raided for their star players late in the window and have no choice but to shop in the final hours before the window shuts. And sometimes they do business late because they think it will yield the best possible deal and, to them, an extra few million pounds either way makes a big difference. But the big boys have the means to get things done early.

It's not an exact science and, like we said, there are plenty of factors that can contribute to teams moving late. There's no doubt that some of the late deals we see are due to circumstances beyond a club's control, or they're calculated risks, which enable a team to save money or gain leverage to get the man they really want.

And yet, you can't escape the feeling that maybe teams underestimate the benefits of getting business done early. And that sometimes, paying a premium is worth it.