The Manchester City striker is at a level to be compared with the likes of Luis Suarez, according to the Ballon d’Or winner
The Argentina team-mates will be on opposite sides at the Etihad Stadium on Tuesday when Manchester City host Barca in the Champions League.
Aguero was controversially named as a substitute for City’s 4-0 loss at Camp Nou in the reverse fixture earlier this month and has not been an automatic selection under Guardiola.
But Messi, who enjoyed sensational success under the Catalan manager during his Barca tenure, has no doubts over Aguero’s quality, ranking him with the game’s top centre-forwards like club colleague Luis Suarez.
“Kun is a very simple and easy-going person, a great lad and, as a striker, one of the best. As is Luis [Suarez],” Messi told The Sun.
“They are different but, if you want to look at it like that, either of them can decide a game. They are both very decisive.”
The Argentina captain feels Barca have tweaked their style of play under Luis Enrique, but does not think things are that different from Guardiola’s reign, which came to an end back in 2012.
“In reality, the system of Barcelona is something that everyone knows,” said Messi.
“I can say we are one of the few clubs that have it, but each boss has different things and applies them as he sees fit.
Messi scored a hat-trick against City in the first meeting and has had huge success against their local rivals Manchester United in the past, scoring in two Champions League final victories over teams managed by Alex Ferguson.
He added: “There are several games when the team played in such a way that it was almost impossible for the opponents to do any damage to us.
“And it almost always happened in big matches. The final of the [2008-09] Champions League in Rome was one of those games when the team played the perfect match.” (Courtesy Goal.Com)