What is a centre-forward? It is a question that is far harder to answer now than it used to be. The suggestion that Pep Guardiola may not be entirely happy with Sergio Agüero seemed at first bizarre. How, realistically, could a player of his ability, his goalscoring capacity – 109 league goals in five seasons at City, despite injuries – be doubted? For a modern striker, though, goals are only part of it.

“It is not about how many goals he scores because we’re happy with the scoring,” Guardiola said this week. He wanted more all-round contribution. Guardiola may be a unique manager but this is not a unique quirk. Jürgen Klopp has made it fairly clear that goals (not that there have been huge numbers of them recently) are not enough to keep Daniel Sturridge in his Liverpool team. Antonio Conte insists Diego Costa should start the process of winning the ball back. This is the nature of the modern vogue for pressing: scoring goals is no longer enough.

This demand for forwards to chase and harry is not new. Almost since football began there have been different visions of how a centre-forward should play. Even Austria in the 1920s – blessed with the deft intelligence of Matthias Sindelar, the Paper Man, one of the earliest incarnations of what would become known as the false nine (although given no one had thought to put numbers on shirts at that point, there weren’t even nines to be false versions of) – went through a long phase of preferring the big tough.

Determining who was the first centre‑forward to be regarded as the first line of defence is almost impossible but the role was essential once pressing had developed in the 1960s. Anatoliy Puzach did it at Dynamo Kyiv, Johan Cruyff at Ajax and Roger Hunt at Liverpool. By the 80s it was common. Ian Rush, for instance, was a master of it.

What has changed is largely a matter of degree, and partly perhaps of sophistication. The widespread availability of video in the 80s and 90s allowed analysts to pinpoint potential weaknesses in opponents, their internal mechanisms. This was Marcelo Bielsa’s great insight, the reason that his first demand on taking the Vélez Sarsfield position in 1997 was a video recorder and software to transfer clips to a computer. How did opponents like to initiate moves? If their default was for the goalkeeper to play the ball to the right‑back to go inside to the right‑sided central midfielder, could his team disrupt that? The increased availability of data and the increased use of computers to discern patterns has made the use of pressing more focused.

Even when Michael Owen was rattling in 40 goals for England, he seemed like an anachronism, a player left behind by football’s evolution. The days of the poacher are over. That has been recognised in football for a couple of decades but the new breed of hard-pressing coach has taken it a stage further.

For a time, wingers were the hardest‑working forwards as they tracked the opposition full-backs. In 2008-09, for instance, after Manchester United had struggled to contain the attacking left-back Aly Cissokho against Porto, Sir Alex Ferguson used Wayne Rooney as a wide‑forward and Cristiano Ronaldo centrally because Rooney was more diligent in performing his defensive functions.

It seems that is changing. By the nature of their position, playing with the touchline to one side of them, it is very hard for a full‑back to be a playmaker. The slightest pressure restricts them to turning inside. A ball‑playing central defender or very deep-lying central midfielder – common at Ajax and Barcelona for some time – can become the fulcrum. And that means there is a need for a centre‑forward who is prepared to hound him.

That is why Roberto Firmino is so valuable for Liverpool. This season he is averaging 11.5km per 90 minutes played, at a rate of 78 sprints per 90 minutes. He makes 3.0 tackles and 0.7 interceptions per 90 minutes. The fact that he also has four goals and an assist is almost a bonus. Firmino is vital because he initiates the press.

Guardiola presumably wants Agüero to do something similar. His figures pale beside Firmino’s (apart from goals, and even in modern football they are important: he has seven in the league this season) with 64.3 sprints per 90 minutes and 9.9km covered, but they are significantly higher than last season when he managed 44 sprints and ran 8.9km per 90 minutes.

That said, he has made only one tackle and two interceptions all season, so he has some way to go before becoming a Firmino. Or perhaps that is not entirely fair: after all, a run towards a player with the ball can force a poor pass that does not necessarily register as a tackle. What is clear is that Agüero’s game has changed under Guardiola and that it has changed even over the course of the season. Sprints and distance covered have both gone up (a little) since he was left out of the starting XI for the game at Barcelona.

The ability to finish, to create space, and to lay on chances for others, of course, remain hugely important, but the renewed focus on pressing has altered the expectations for the centre-forward.