Exhausted Romelu Lukaku at the root of Manchester United and Jose Mourinho's problems
A former coach once used a straightforward drill to illustrate the effect of tiredness on a player's concentration and technique.
Firstly, every player was given a ball and told to count the number of keepy-uppies they could perform. After that came one of those sprint sessions that burns the legs, fogs the mind and leaves the player barely able to speak. At that point, they're thrown the ball and count the number of keepy-uppies again.
There's nothing different about the task, and nothing physically exerting about keeping the ball off the ground, but invariably - unless you only managed four the first time around - the number drops once the tiredness kicks in.
All of which might help to explain why Romelu Lukaku's ability to link up Manchester United's attacking play in the course of the last month has, in the crueller part of the footballing analysis world, been compared to a four-legged animal.
One of the most repeated videos of the 0-0 draw against Valencia came in the early stages of the second half when Lukaku found himself on the left wing, just inside the opponent's half trying to reach for a ball and build an attack and some momentum for a team that had been booed off at half-time.
Instead, Lukaku couldn't get his foot wrapped around the ball and, rather than team-mates racing to get up in support, it was the ball boys around the sideline at Old Trafford who were being worked as the ball clunked off Lukaku's foot and went out for a throw-in.
It's easy to mock such moments which can happen to anyone at an unfortunate moment but was typical of a player who is a step away from his prime form simply because he looks exhausted. And with good reason.
Since August 8 last year, when he made his Manchester United debut against Real Madrid, Lukaku has been flogged.
From that date until the game against Valencia last Tuesday - which adds up to 60 weeks or 420 days - he has appeared in 72 games for club and country which, to save the reader a maths test, tallies to a game every six days for just under 14 months.
And that, supposedly, is meant to include a break between the World Cup and beginning of the Premier League season in which some players had the opportunity to recharge their batteries.
Lukaku, increasingly, resembles one of those faulty phone batteries that will charge fully when plugged in but, then drops into the red at an alarming rate.
In 56 of those 72 games, Lukaku has completed the full 90 minutes which, with an average of five minutes of injury time added on, means that in those games alone Lukaku has been on the pitch for three and a half days.
The regular but nonsensical argument put forward in this scenario points to the amount of money that a player earns in that period which is fine for his bank balance but it's an odd approach from a manager and club to spend so much money on such an important player and then drive them into the ground.
A year ago, Lukaku was coming off a stunning start to his United career with his goal in the 4-0 win against Crystal Palace at the end of September bringing his tally to 15, albeit with the caveat that one of those games was against Gibraltar and another against a Slavan Bilic-managed West Ham.
And yet the warning signs were there with Lukaku having played every minute of the first 12 games of the season including times when it would have been just as easy to give him a small break.
At 2-0 up against Basel with 20 minutes remaining, Mourinho took off Anthony Martial; when it was 3-0 after 26 minutes against CSKA Moscow, Lukaku still remained until the final whistle as he did when it was the same scoreline against Palace with 40 minutes remaining.
From the end of September, after 12 full matches in seven weeks, Lukaku scored just once in the following two months. There was something of an improvement in December but this may have been down to the law of averages as much as anything with Lukaku featuring in nine games in 28 days.
Included in this was a 29-minute cameo off the bench in a desperate attempt to help United avoid an embarrassing defeat to Bristol City when a line-up containing Paul Pogba, Zlatan Ibrahimovic, Anthony Martial and Marcus Rashford still needed Lukaku to attempt to rescue the situation.
That he couldn't do it on this occasion was a far greater indictment of Mourinho's management than of the players because he created a system that is so reliant on Lukaku that if he's not there, or is out of form, the whole thing grinds to a halt.
At Manchester City, Sergio Aguero and Gabriel Jesus are different in personal style but don't radically shift the team's approach; it's a similar story at Liverpool where Daniel Sturridge can slot in reasonably well for any of the front three while Chelsea use either Alvaro Morata or Olivier Giroud as their focal point.
At United, when it comes to the point of attack, there's Lukaku and then there's nobody because to change striker means altering the team's entire approach, unless the option is to stick Marouane Fellaini up alongside him.
With City, Liverpool, Chelsea and Tottenham, changing their front players means a slightly-refined Plan A. At United, playing Martial, Rashford or Alexis Sanchez instead of Lukaku involves a completely different style and everyone is suffering because of it.
For a player of his size, holding the ball up isn't particularly a strength of Lukaku's in the manner of a Didier Drogba, Alan Shearer or Mark Hughes, who could simultaneously stick out their backsides to shield a defender, control the ball and have the awareness to bring team-mates into the game.
Lukaku's attributes are more based around his explosiveness in which he runs down the channel of a defender or spins the ones that get too close in order to get clear on goal. This, however, requires both good supply from the players behind him and movement from the player himself, neither of which has been happening in recent weeks.
Rio Ferdinand was particularly critical of Lukaku's movement during the stalemate against Valencia, describing the lack of it as "a defender's dream".
"When the ball is developing more in central areas of the pitch he just stands and really he's got to be moving even if he's sometimes standing offside," said Ferdinand. "Make defenders think, and at the moment defenders are seeing exactly where he is, he's not moving so you know what he's doing and for a defender that's a dream."
Ferdinand's points are absolutely valid but what he is describing sounds like the sort of sharpness that it's difficult to have when Lukaku is the only player in the United squad who has played every minute of the six games in 18 days in which the team as a whole are toiling.
Unlike a Drogba or Harry Kane, Lukaku doesn't seem to have the sort of body shape which can take the punishment of those kind of demands while still staying as sharp as the team need him to be. At the moment, he looks heavy-legged and a little heavy-set.
There is some rest on the horizon for Lukaku after this evening's game against Newcastle with a mammoth 10-day rest period before Belgium play Holland in a friendly on Tuesday week. For context, that's over one-third of the 26-day period which Lukaku got to rest between the end of Belgium's World Cup campaign on July 14 and the start of United's Premier League season on August 10.
But, if Mourinho is still in a job after the international break, there's little doubt who he will turn to in the hope of digging him and the team out of their slump.
"For many months you are always asking me why this player doesn't play, why that player doesn't play, why this player is on the bench, why always Lukaku?" asked Mourinho last May as he used Lukaku to criticise the likes of Martial and Rashford.
Within three weeks of that relative praise, Lukaku was on the end of Mourinho's passive aggressiveness as he aimed a dig at his admission that he wouldn't be fit for the FA Cup final.
"He made a decision - not ready to start. Maybe to give us a little help… it's one thing to start the match, another to play a few minutes. That's what he told us he would be ready to do," whined Mourinho before the defeat to Chelsea.
Lukaku might justifiably have pointed out that, for example, bringing him on for 25 minutes against Yeovil when United were already 2-0 up on the way to that FA Cup final might not have been the best way to manage a player expected to shoulder such a mammoth workload.
It would be helpful for Mourinho were Lukaku to suffer an old-school Ryan Giggs-like hamstring niggle on the eve of an international break against Newcastle this evening but such is the reliance on him, and Mourinho's inability to alter his style to suit alternatives, that Lukaku will be the focal point again in the hope of flogging him and the team into form.
It might work for a little while but, as the games come faster, it's difficult to think that Lukaku will be able to keep going all the way until May for what could be well more than 100 games in two seasons with a few days' rest somewhere in between. Like most things in Mourinho's third season, or keepy-uppies after a series of sprints, there's only so much of it a player's body can take before it eventually just gives up.