Pep Guardiola believes Sergio Agüero is “irreplaceable” and filling the void when the striker leaves Manchester City will be one of his toughest managerial challenges.

Agüero is City’s record scorer with 244 goals, yet the 31-year-old striker is set to leave in summer 2021 when his contract expires. Guardiola has worked with some of the world’s finest strikers and he was emphatic when offering his view of the Argentinian.

“Sergio is irreplaceable,” Guardiola said. “It will be one of the toughest asks of my career to [replace him]. We have another incredible striker in Gabriel [Jesus] and maybe another one will come when Sergio leaves but he is irreplaceable.

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“The most important moment of this club – more important than when this club hopefully wins the Champions League – is when Sergio scored that goal against QPR [to win 2011-12 title]. That moment is much better than four titles in one season [of last term] or whatever and he was the guy.

“Sooner or later it will happen; he’ll decide when [he leaves]. I’m sure the club will be ready to try to find the guy to replace him but of course it will not be easy.”

Agüero is again City’s top scorer in the league, with nine goals, and his total of 173 is sixth in the competition’s all-time chart.

Guardiola said: “He’s done incredibly well. I’ve never seen such a big star like him be so humble, so funny. It is a joy to work with him and how he accepts my decisions when it sometimes doesn’t work for him.

“Big stars don’t [usually] accept that. I’ve worked with other stars who believe they were bigger than Sergio when they weren’t and they were more difficult to handle and be with. It’s not easy to find a guy with his status, his personality, with what he’s done in his career and have his humility, accepting everything perfectly.”

Facebook Twitter Pinterest Sergio Agüero’s last-gasp title-winning goal in 2012 – a moment Pep Guardiola has called the most important in Manchester City’s history. Photograph: Tom Jenkins/The Guardian

Guardiola was asked if Agüero could be persuaded to extend his City career beyond 2021. “I don’t know. He can stay but it depends on his physical condition and desire. I haven’t spoken with him and I don’t know his intentions. Sometimes you [the manager] have to take a decision but with most of the cases the players decide. I would like Sergio to decide whether he wants more or it is over.”

The manager rates him below only Lionel Messi as a finisher. “In the box he is incredible but I had other guys who were not bad. Ronaldo, the Brazilian, Romário, [Samuel] Eto’o – he was one of the most incredible strikers I’ve seen in my life, he could do everything – and Sergio is of course one of them. Messi is a little bit higher than them but Sergio is one of those below.”

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Guardiola cannot say whether Agüero will be able to sustain his levels next season when he is 32. “The decisions being made right now are about the game we’re going to play. There are moments that Gabriel is much better or Sergio is much better or I take the decision [who to play].

“But thinking about next season and Sergio being 32 and playing Sergio or Gabriel is unrealistic. When Raheem Sterling is injured he doesn’t need any time to get fit, maybe half a day. Sergio needs a little bit longer for his physical condition. That is the reality.

“We will give him as many minutes as possible because we need him for the next few months and the Champions League as we need all the players.”

Agüero has recently recovered from a thigh problem and could make his first start for a month in the trip to Wolves on Friday.