Messi's contract is also set to run out next month and his future has been shrouded in speculation since he first said he wanted to leave Barca last August.
However, Aguero, Messi's teammate for Argentina, said at his presentation that he believes the two friends will be playing alongside each other at Camp Nou next season.
"We hope to play together, of course," Aguero told reporters when asked if his arrival was an indicator that Messi had decided to stay at the club.
"But what happens with Leo is a decision he will take with the club. It's a pleasure to play with him. If he stays here, which I think he will, we will give our best for the club to achieve big things."
Aguero and Messi came through the ranks together with Argentina, winning the Under-20 World Cup in 2005 and the Olympic Games in 2008, and have remained close ever since.
"I speak with [Messi] every day, but I can't tell you what about," Aguero added, laughing. "The last message was him congratulating me on my signing.
"I've known him since I was a kid, but training with him every day, because the national team isn't that often, is going to be much easier."
ESPN has reported that Barca president Joan Laporta is working on a two-year deal for Messi, who turns 34 in June, with the option for an additional season.
Laporta denied, though, that the signing of Aguero was a sweetener to help convince Messi to stay.
"We've signed [Aguero] because he is an exceptional player," Laporta said at the presentation. "He plays in a position which is very important for any system and especially in ours.
"He is clinical and that's important. We've signed him because we want a competitive team. We also want Leo to stay and he knows that. [Aguero] is the first in a series of signings that will be announced throughout the month."
It remains unclear if Ronald Koeman will be the coach in charge of the new signings, though. Aguero said he has not yet spoken with the Dutchman and Laporta added that they're currently "reflecting" on what to do.
"We've opened a period of reflection with Koeman," the president explained. "We have to assess what's gone well and what hasn't gone well this season.
"We need this period of calm and in a week or 10 days we will meet again, but there's contact. We want to respect the contract he has and talks are going well. This reflection is necessary."
Koeman's deal runs until 2022 and reports have suggested Barca would have to pay €8-12 million to buy out the reminder of his contract.
Multiple sources told ESPN earlier this month that Barca are exploring replacements for Koeman, but Laporta doesn't want to sack him without having a successor in place.