Despite their draw against Atalanta at San Siro, Manchester City top their Champions League group and need only a point from their two remaining games to secure a place in the last 16. Perhaps this will be the season they win the one prize that has eluded them during their massively expensive modern overhaul. But as Pep Guardiola prepares for a meeting with the only English club that has come even close to rivalling the recent domestic achievements of his City side, he must be aware his repeated failures in Europe are starting to significantly dent his reputation. If he is going to turn this situation around, maybe this has to be the time.

Guardiola has guided City to the league title in each of the past two seasons, winning an astonishing 198 points along the way, plus three domestic cups and a couple of Community Shields. In Spain, Germany and England he has won the league in eight of the past 11 years. It is an incredible record, one that puts him in the very top category of management. But before a manager can be defined as great, what must they achieve?

If greatness is defined by how many times a manager wins the greatest prize in the sport – the Champions League – I don’t think Guardiola qualifies. If it is defined by what you achieve with limited resources, Guardiola has never been tested. If it is defined by the ability to transform the fortunes of the clubs they manage, then Guardiola – who took over Barcelona when they had won the league in two of the previous four seasons, Bayern Munich when they were reigning champions, and Manchester City when they had been champions in two of the five previous seasons and had unrivalled spending power – is unproven. He can coax the already great to new levels of achievement, but is that enough?

Jürgen Klopp has a fraction of Guardiola’s medal collection but he took second division Mainz into Europe for the first time in their history, took over a Borussia Dortmund side who had finished 13th in the Bundesliga and led them to consecutive league titles and a Champions League final, and has transformed Liverpool into a machine that won 97 points last season, stands six points clear as they prepare to welcome City to Anfield on Sunday, reached successive Champions League finals and won the more recent. He has won two league titles, both with the same club, but in many ways his record is the more complete and more impressive. And it is his success in Europe that really sets him apart.

Jürgen Klopp and Guardiola will face each other once more on Sunday. Photograph: Jason Cairnduff/Action Images via Reuters

Perhaps it is unfair to hold Guardiola’s lack of European achievement against him. After all, City were knocked out of last season’s Champions League by Tottenham in the quarter-finals after VAR disallowed Raheem Sterling’s stoppage‑time goal for a marginal offside and allowed a Fernando Llorente strike that might have been chalked off for handball. If a couple of calls had gone the other way, refereeing decisions that Guardiola could have no control over, it could have been their year but instead they were out on away goals. But it was not a one-off: it was the third time in four seasons Guardiola sides had been eliminated on away goals. In Europe’s premier competition the Spaniard’s teams have repeatedly failed to come through close games. As Liverpool showed against Barcelona and Tottenham against Ajax in last season’s semi-finals the best teams find a way to win and City could not do that.

Guardiola has never got to a European final without Lionel Messi, whereas Klopp has won one with Divock Origi. Guardiola’s Barcelona side won the treble of La Liga, Copa del Rey and Champions League in 2008-09, something no other manager had done, but Luis Enrique repeated the feat in 2014-15 and nobody has him down as one of the all-time greats. José Mourinho won the Champions League with Porto and Internazionale and eight league titles in four countries, yet he seems to be disregarded as an old‑school manager with outdated ideas.

Guardiola’s achievements put him in the highest level of European management but though his Barcelona team probably have to be counted among the greatest in the sport’s history in my opinion the title of genius has been given to him too quickly, particularly when there is another manager in the same league at the same time who has perhaps overtaken him in terms of his effect on his club and his achievements with them.

Some people seem to think Klopp is first and foremost a motivator, whose teams are reliant on energy, power and pace, where Guardiola is a tactician. I think this is a completely unfair way of characterising what the German has created at Anfield. A lot of money has been spent in the transfer market but in a strategic and deliberate way, forming a front three who rival any attack in world football, bringing in probably the best defender on the planet in Virgil van Dijk and at the same time promoting young talent and making the most of undervalued players from other English clubs such as Andy Robertson and James Milner.

He is also extremely consistent in tactics and team selection, where Guardiola is sometimes guilty of overthinking, both tactically and in the transfer market, signing incredibly expensive substitute full-backs and tweaking his formations, particularly in big games.

Given Guardiola’s success as a manager, it seems bizarre to say he is under any pressure except whatever he chooses to place on himself. It is ridiculous even to consider the possibility of him getting sacked but he will not be at City for ever and whether he chooses to leave or is eventually told to go, what are the time limits and what does he need to achieve before he departs? At what point does that pressure really start to build? And if City were to once again be edged outin the quarter-finals this season, where would that leave him?

City will be desperate to win at Anfield but the victory Guardiola truly needs to cement his reputation cannot come until May, at the Ataturk Stadium in Istanbul, the venue for the Champions League final.