There was an interesting moment towards the start of Jürgen Klopp’s time at Liverpool, back in the days when he still felt the need to apologise for his already-excellent if amusingly sweary English. Asked about transfer targets, budgets and spending big on new players, Klopp didn’t really answer the question. Instead he grimaced and grumbled, rubbing his eyes behind his glasses like a weary, drink-sozzled chief inspector with a short fuse but a heart of gold being told by one of his hair-gelled sergeants that there are no suspicious circumstances so should he close the case on the mysterious death of Lord Sinister.

Klopp went on to talk about trusting his existing players, about chemistry and coaching and the moral obligation to improve what he had rather than team-build by numbers. Six months later, more confident in his systems, not to mention his command of English metaphor, he came up with the train analogy to describe his idea of a team as something entirely collective, fired by a shared momentum as opposed to opportunist chop and change.

“If I spoke to a player and he told me: ‘If you were playing in the Champions League next year then I would be really interested,’ I would put the phone down. That is what I would say to players. It is about pushing the train, not jumping on a running train.”

Liverpool supporters have had a week to worry away at the question of how to replace Philippe Coutinho, how to fill the obvious gap his departing star quality will leave. But as the club adjusts to losing its best player for the third time in four years it is worth remembering that there is a difference now.

From Dortmund to Liverpool, Klopp’s notion of team-building is designed to survive exactly this, the vicissitudes of life as a selling club at the highest level, of success always placing your best players in the crosshairs of those with bottomless funds and the promise of a kind of club football ultimacy in Madrid or Barcelona.

This is the way Klopp’s Liverpool are geared to work. Soft-shoed Brazilian playmakers will come and go, but unlike Brendan-Luis, or even Brendan-Raheem, the manager is the real star here, the one irreplaceable part around which all progress revolves.

The most important person at the club is still in place. Klopp isn’t just in the cab, shovelling his furnace, tooting the whistle. He’s the guard, the engine, the coal scuttle, the pressure gauge. Klopp is the train.

With this in mind, the visit of Manchester City on Sunday is unexpectedly timely. In a way Pep Guardiola’s league leaders are the ideal opponents. City have crossed over into another place in the last few weeks, the most dominant leaders in any major league, 15 points clear of second place before the weekend’s fixtures and with further clear blue sky opening every week.

Sadio Mané’s kick to Ederson’s head resulted in a red card that sparked Manchester City’s 5-0 rout of Liverpool in September. Photograph: Lee Smith/Action Images via Reuters

In effect City’s role right now is to act as a kind of control experiment for the rest of the league, a point against which every other ambitious team must for now measure itself. The interest for the neutral lies in how each opponent will try to cope with City’s width and fluency, their mastery of the ball, as much as it does in another frictionless demolition.

Klopp has an opportunity straight away to demonstrate the reserves of strength left in his squad, and also to reaffirm his own capacities, that ability to keep the train running whatever the snags and jolts along the way.

There is already a sense that Sunday could be a game of fine details, or at least something more refined than Liverpool’s oddly flaccid 5-0 defeat at the Etihad in September. Liverpool were already losing that game when Sadio Mané was sent off but their subsequent collapse crystallised the idea of this team as defensively flaky free-wheelers, City as a kind of surging, pirouetting goal machine.

Things have changed a bit since then. City’s run of form has narrowed. This is a team marked out currently as much by its control as its rapier attack. A defence that was once widely snarked at has not conceded more than one goal in its last 14 domestic matches, and has not conceded more than two in a domestic game in exactly a year.

Meanwhile, Liverpool have been on a wonderful run of their own, with no defeat in 17 since the shellacking by Spurs at Wembley. Coutinho was involved in a quarter of their 54 goals in that time, despite playing only 11 matches. But the team have produced excellent results without him. Coutinho didn’t play in the defeat of Manchester City a year ago. He didn’t play in the 4-0 devouring of Arsenal this season.

It might seem odd to suggest the departure of Liverpool’s best player is a chance to show the progress made under Klopp. Those who are innately hostile will point to the absence of a title challenge, an objection known as the no-better-than-Brendan delusion. But Klopp has burnished his reputation, has pulled together the seams, has shown he can call a club to order.

Liverpool are currently en route to a second successive season in the top four for the first time since 2009. That fluent, fun, workaholic attack is a genuinely notable creation. If it feels as though the team are being built from the front, then at least something is being built. Best of all Anfield still seems energised by Klopp, at ease with itself, a place the manager really does seem to fill and feed off and nourish with his presence.

Klopp has kept his word, too, creating a team out of high-grade component parts rather than star signings. Liverpool’s starting XI in their last Premier League game featured six players Klopp inherited. There have been some post‑inflationary signings but under Klopp £225m has been spent to date and £270m raked back.

Right now, the biggest challenge is not so much to replace Coutinho as to ensure the aftermath is managed to Klopp’s satisfaction, that their most valuable asset remains settled. Christian Purslow might not be everyone’s idea of an all-seeing footballing sage but there is something in the observation by the club’s former managing director this week that Klopp remains “absolutely A-list… the next manager for one of those bigger clubs if Liverpool do not fulfil his ambitions”.

Sunday is another opportunity to show that train is still running on, that this is a team geared to absorb a high-level departure without losing their head of steam. There have been various tactical offerings in the continuing mystery of how to go about derailing this Manchester City machine. For a while in autumn the idea of sitting deep and trying to disrupt the midfield fulcrum of Fernandinho and Ilkay Gündogan gained some traction, although this always felt a bit like placing your neck on the block and waiting for the blade to fall.

Roy Hodgson’s approach at Selhurst Park on New Year’s Eve was successful, as Crystal Palace succeeded for 45 minutes in applying genuine pressure to City’s weak points, pressing hard in midfield and isolating City’s defenders one on one against Wilfred Zaha. This is also the most natural approach for Klopp’s Liverpool, with a mobile, powerful midfield and a hard-pressing front three pushed right up against City’s defenders. The absence of Coutinho hardly hampers this system.

Klopp might just feel this team, with the midfield fury of Naby Keïta to come from RB Leipzig, is a little closer to a vision of the pure Klopp style. Either way the Premier League could do with at least one desperately fought top-of-the-table match before the season turns entirely Sky Blue. Sunday at Anfield could yet be the moment.