Manchester City have kept two clean sheets all season. Liverpool have kept three. Both sides may have come out of last weekend lamenting defeats that have allowed Chelsea to open a small gap at the top of the table but it could be argued that with defences like they have it is remarkable they are as high up as they are. But this, perhaps, is the modern way: defending in the old-fashioned sense has become unfashionable.

“There is no modern generation,” José Mourinho growled in the summer of 2015 after winning the Premier League with Chelsea. He was responding to criticism of his supposedly “boring” style of play and with a certain level of wilful misunderstanding dismissed those who supposedly rejected the counterattack as “stupid”.

He may have had a point that there are certain coaches who pay too little attention to their defences but he was surely wrong about there being no new generation. Pressing, high lines, percussive, vertical football are in vogue, from Dortmund to Liverpool, from Sevilla to Hoffenheim.

There are differences. There are the Cruyffians, the Bielsistas, the German school inspired by Valeriy Lobanovskyi via Ralf Rangnick, and the post-Arrigo Sacchi Italians all using variations on the basic idea of pressing high and it is they, for all the successes of Mourinho and Diego Simeone, who represent the essence of modern football. And that, seemingly, means goals are on the up again.

In the Premier League, there was a clear shift in around 2009, from a situation in which 2.5-2.6 goals per game was normal to one in which 2.7-2.8 was. Over the past couple of seasons there was retrenchment but this season goals are up again to the extent that if the average is maintained it will be the highest-scoring season in 20-team Premier League history.

The pattern is more complicated in the Champions League but the recent trend is upwards, to almost three goals a game in the group stage.

There is very rarely a single, simple explanation for anything. One of the reasons goals-per-game has gone up in the Champions League is that the gulf in quality between the best and the worst sides has increased and so there are more hammerings.

That might perhaps have happened anyway given the economics of modern football as the wealthy continue to get wealthier but it’s a process that has been exacerbated by the introduction of the Champions Path to qualification in 2009 which, however well-intentioned, has led only to more mismatches in the group stage.

A decade ago there were usually four or five games of the 96 in the group stage that ended with a team winning by four or more goals. Now there are commonly 11 or 12. This season, with eight games to play, there had been a record 13.

That is not the case in the Premier League, where it is hard to discern any pattern in the number of victories by four goals or more – other than to point out how few there are by comparison with the group stage of the modern Champions League, bearing in mind there are four times as many Premier League games. So what does lie behind the rise in Premier League goals?

The step in 2009 seemed to stem from two inter-related reasons: a desire to ape Pep Guardiola’s Barcelona and play more progressive, open football, something that was facilitated by the liberalisation of the offside law, which effectively killed the offside trap as a systematic tactic, forcing defensive lines back and so creating a greater playing area in midfield. Even promoted sides were swept along by it.

Whereas the tendency had been for teams to sit deep, scrap away and try to nick a goal on the break, particularly away, sides such as Blackpool and Swansea looked to have the ball and attack. More recently those defensive lines have crept forward again, without such protection from the offside law, and physical contact in the midfield has increased. There has been a gradual shift in focus from technique back towards physique, which is why the most significant figure in the Premier League over the past season and a half has arguably been N’Golo Kanté.

It would be an over-reaction to write off Guardiola’s purism but equally it is notable how his sides have – occasionally – been picked off by rapidly countering opponents: what Chelsea did to them on Saturday recalled what Real Madrid did to Guardiola’s Bayern in the 2014 Champions League semi-final.

But it is not just to do with defensive lines being high. The focus on passing the ball out from the back is fraught with danger – see Steve Cook for Bournemouth against Arsenal, or John Stones for City against Southampton – but it also means a change in emphasis. Defenders and goalkeepers are no longer selected for their capacity to stop opposing attackers but also for their passing ability and that seems to have led to a drop-off in the basic skills of negation.

The flip side is defenders who lack technical ability can be exposed by an opponent playing a high press; this is not a problem that can be rectified by a return to a more old-fashioned style of defensive defender, unless he is also good on the ball.

In that sense, the increase in goals is largely an issue of tactical philosophy. The change in the offside law permitted a style of flourish, and that style, in its various guises, has become hegemonic.

That in turn has had a major impact on the type of players sought by top clubs. Guardiola’s version of the approach has been challenged recently by a more physical, more vertical variant but it is still part of the same proactive, high-pressing school.

The Guardian Sport