Ronald Koeman has said Everton’s most important move of the summer was convincing Romelu Lukaku to stay but believes the striker will be coveted by European heavyweights such as Bayern Munich and Real Madrid should the 23-year-old continue to improve.

Lukaku returns to his former club Chelsea on Saturday having scored seven of Everton’s 15 league goals this season and assisted in three. Chelsea wanted to re-sign the Belgium international in the summer but, having sold him for £28m in 2014, refused to countenance Everton’s prohibitive £60m-plus price tag. The striker admitted this week that he was encouraged to remain at Goodison Park by an “honest” conversation with Koeman shortly after the Dutch coach had replaced Roberto Martínez as manager.

Koeman, however, insists the final decision rested with Lukaku and feels the striker has the potential to join the European elite.

The Everton manager said: “In my position he was and is a very important player for the club and I did everything I could to keep him. But the final decision is down to the player and that’s what happened. It was the most significant thing that happened in the summer. We try to be prepared because he will not play [for Everton] for always. He is a big striker who will always generate interest from big clubs and from bigger clubs than Everton but the boy is 23 and he took the right decision to stay.

“He can improve and he needs to improve. That’s good because it is very difficult to get a replacement for a player who compares to Romelu, maybe impossible. I was a little bit afraid to lose the player. Yes, we would have the money but we don’t have a striker who would score 20-25 goals. It can be the most difficult situation but we got the decision and everybody was happy.

“Maybe we will get the same problem next season. I have read some stories about [moving in] January – no way, no way! Bayern Munich, Juventus, Madrid, Barcelona – he stays until the end of the season at least. OK, that needs to be the next step for him and then we as Evertonian people can be proud that he made that step by scoring goals. That’s the best answer he can give on the pitch.”

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Koeman has identified room for improvement in Lukaku’s pressing game but believes the former Anderlecht and Chelsea player is already one of the most complete strikers in the Premier League.

“He’s a strong man in front, he’s fast and one of the most clinical in front of goal,” Koeman added. “We do a lot of finishing sessions during the week and he’s impressive in how he scores and how comfortable he is in the last part of attacking. He will get more experience and he will get stronger. We try to do better pressing but that is a little bit more difficult because he needs to know when to run and when to start your run and to improve in every aspect of a football player.

“But he is strong, he has the movement, he can keep the ball and he’s one of the best. I had [Graziano] Pellè at Southampton who was very strong and tall, really fast with the feet and to move, but Rom is a better finisher than Pellè was. He is one of the most complete attack strikers we have. You can play a long ball to him. To play a long ball up to [Sergio] Agüero is more difficult but they are top strikers and Rom is only 23 years old and the best needs to come.”