Aguero returned to training with City on Wednesday and could potentially be back for the Premier League clash with Stoke City on Saturday.
Argentina's national-team doctor was quoted last week saying Aguero was likely to be sidelined for up to six weeks, but the player himself appears to have the initial prognosis in mind.
Messi has a new deal on the table from Barcelona worth around £500,000-a-week.
"I still do not know if I am going to play on Saturday", Aguero told TyC Sports. He said, "I don't believe so.' I took a moment to think and realised it's thanks to wearing a seatbelt that I'm able to talk about this at all".
"Then one thinks 'why did I?', but it's hard to imagine that something like that could happen".
Meanwhile, City are wary of rushing back the 29-year-old too quickly, as they are conscious that a further blow to the ribs could cause more damage if the injury has not healed properly, and will leave a decision on his availability as late as possible.
Aguero spoke to Messi after his hat trick for Argentina secured qualification for the World Cup.
"I don't know yet". "On Thursday I will train with the group and see if I'm comfortable".
Meanwhile, Aguero dismissed reports that he is set to join AC Milan, with City said to be chasing Arsenal forward Alexis Sanchez.