The previous paragraph probably applies to Zlatan Ibrahimovic more than anyone else, but there is an arguable case that it also applies to Ander Herrera. The Spain international has been routinely magnificent, and has been the constant and understated presence in United's superb run of form over the last two months.

While Ibrahimovic, Paul Pogba, Henrikh Mkhitaryan, Juan Mata and Michael Carrick have drawn most of the plaudits, Herrera has run busily alongside them. If Zlatan is the F1 driver, then Herrera is the pit-lane engineer. Last weekend he snapped and snarled his way through the midfield, balancing his forceful performance on the edge of a second yellow card, and willed his team to victory in a manner of which Roy Keane might have been proud.

It is easy to be fooled by Herrera when you first look at him. He is not particularly tall, and his fresh-faced demeanour suggests that of a polite and diligent postgraduate student, possibly studying economics. He looks like -- well, like a nice boy. But he is assuredly not a nice boy. Nice boys do not conjure slide tackles with the viciousness of bear traps. Nice boys do not steal possession as if it were a wallet left too long on the adjacent dinner table. Herrera is an unlikely source of fury, a jackal disguised as a choirboy.

It is when his statistics are placed alongside those of other leading Premier League midfielders that his remarkable impact becomes clear. Chelsea's N'Golo Kante is rightly regarded as one of the key factors in his team's title challenge, the France midfielder getting through a prodigious amount of work each match. Yet Herrera must protect a defence less secure than Kante's and must supply an attack less fluid, and that is reflected in the numbers.

Per 90 minutes in the Premier League, Herrera creates more chances (1.43 to 0.64), makes more interceptions (3.46 to 2.33) and makes more tackles (2.57 to 2.37). He may also be more susceptible to wear-and-tear, suffering far more fouls (1.68 to 0.44) but is clearly no saint himself (making 1.83 fouls per 90 minutes to Kante's 1.57).

Though it is great for United that Herrera is enjoying an outstanding season, Jose Mourinho must already be turning his thoughts towards reinforcements in this area. In short, Herrera needs help. Those numbers may sit proudly in comparison to Kante's, but they also contain a warning. Kante is so effective precisely because he is not being overworked, and there is a danger that the intensity of Herrera's game may take its toll.

Ander Herrera has enjoyed a stellar season at Manchester United.

Pogba, for all of his extravagant gifts, is not a footballer with particularly keen defensive instincts, and Carrick, though a magnificent dictator of play, does not have the mobility that would help Herrera to keep down his mileage.

There is also the small matter of what happens whenever Herrera does not play, which is similar to what happened last season when Barcelona were without Ivan Rakitic -- the tempo of both the passing and movement were notably more sluggish. If anything, it could actually be a fine thing if Herrera's statistics dropped slightly next season, as it might be a sign that the balance of responsibilities in midfield was substantially better.

For now, though, what he is doing should be celebrated. He is arguably as cerebral a midfielder United have had in recent years, having played under Ernesto Valverde, Marcelo Bielsa and Louis van Gaal. He also takes to his play with the passion of a United fan -- a quality which finds itself in the performances of his fellow Spaniard, Mata.

Mata did not have the best of days against Southampton, but the passing patterns he weaves with Herrera have often been devastating for the opposition this season. Indeed, if there is anything that Herrera can add to his game, then it is the odd extra goal here and there -- he has shown that he can be a fine finisher when called upon, and with so much attacking talent he can benefit from the preoccupation of defenders by arriving late in the area.

Yet this is all fine tuning of a machine in near-perfect working order. It has taken Herrera longer to get here than he might have hoped -- far longer than he should have expected, perhaps -- but he has finally become pivotal to United's trophy challenges on all four fronts, and looks to remain at the heart of their push for silverware for years to come.