Paul Merson has expressed his surprise at the announcement of Sergio Aguero's departure from Manchester City this summer but believes that Pep Guardiola must already have a replacement lined up.

City announced on Monday evening that the club's all-time record goalscorer is to leave the club at the end of this season when his current contract expires, after a hugely successful 10 years at the Etihad Stadium.

Former Arsenal and England midfielder Merson believes City must be lining up one of the "big three" forwards in world football – Borussia Dortmund's Erling Braut Haaland, Tottenham Hotspur's Harry Kane or Paris Saint-Germain's Kylian Mbappe – as Aguero's replacement.

"I am shocked at the decision, but Manchester City must have someone else in mind. They must be bringing in a big name like an Erling Haaland, a Harry Kane or a Kylian Mbappe. Otherwise, how do you not keep hold of Aguero? It's impossible not to keep him. He's a goal machine!" Merson told Sky Sports News.

"I can understand that the injuries he's picked up over the last couple of seasons have played a part, but if you are not bringing in Haaland, Kane or Mbappe, you keep Aguero. Even if he just plays 20 games, he's getting you 15 goals.

"I can only think that one of the big three, Haaland, Kane or Mbappe, are coming in at Man City next year, and City are thinking that they will be playing every week, Gabriel Jesus will be the backup and it is disrespectful to Aguero to have him as your third choice striker."

Aguero, 32, has helped City win four Premier League titles, five League Cups and an FA Cup since joining the club in 2011, and there could be more silverware yet to come with Guardiola's side still chasing a quadruple this season.