MANCHESTER -- Manuel Pellegrini believes striker Sergio Aguero is one of the top five players in the world and feels he is not far behind the dominant duo of Cristiano Ronaldo and Lionel Messi.
The in-form Aguero took his tally this season to 17 goals in 18 games with a hat trick as Manchester City beat Bayern Munich 3-2 in the Champions League on Tuesday.
Between them, Messi and Ronaldo have won the Ballon d'Or in the last six years and City manager Pellegrini, who coached the Portuguese during his season in charge of Real Madrid, feels Aguero is closing in on them.
And he challenged the Argentina international to become the best, saying in a news conference: "We are always trying to demand to Sergio to fight always to be the best player in the world because he can do it.
"I am sure he is in the top five. I don't think he is better than Cristiano or Messi but he must be very near."
Aguero has scored 14 of City's last 23 goals and they have been accused of relying on the former Atletico Madrid forward too much.
And Pellegrini, who only has two fit strikers, admits it would be difficult to find an alternative to him, but pointed out that the 26-year-old suffered from injuries last season and City still became champions.
"I think than no one can replace Messi at Barcelona, Ronaldo at Real Madrid or Sergio here. It is not the same to play with Aguero or without Aguero," he added. "But I always say we have a squad and if for different reasons Sergio cannot play, we must have another one that can do it. If you remember, we won the title here last year with a lot of games without Sergio but when he plays, it makes a difference."
Aguero is set to make his 100th Premier League appearance when City travel to Southampton on Sunday.
Only Alan Shearer and Ruud van Nistelrooy have scored more goals in their first century of appearances in the division than Aguero, who has struck 64 times.
Pellegrini said: "It is difficult to compare careers of very important players. Alan Shearer and Ruud van Nistelrooy are players who have already finished their careers. Sergio is not starting but just 26 years old. He has many years more to play here.
"I am always asking and demanding that he tries to be the best player because he has all the conditions to do that. That is why I am very glad for him he is always improving every day."
Pellegrini believes Aguero is remaining fitter this year because he was eased into the season with substitute appearances.
He added: "I think that this year we prepare him in the correct way because if you remember in the first three games including Community Shield, we didn't play with Sergio.
"We just gave him 20-25 minutes, no more, so he could have a good preseason because he needs that after the last season when he has a lot of injuries."