Aimar, who only made his playing return to boyhood club River Plate in May, managed two appearances with the Buenos Aires-based club in his second stint due to injuries, before calling time on his career on Tuesday.
After being told by coach Marcelo Gallardo that he would not be included in the extended squad to be used in the semi-finals and - potentially - final of the Copa Libertadores, Aimar decided to hang up his boots.
"Yesterday they advised me that I will not be on the list of the Copa [Libertadores], and I understand," the 35-year-old wrote in a letter to his team-mates.
"I do not want to occupy a place that probably is for other guys. So I decided stop playing professionally."
Without Aimar, River defeated Guarani 2-0 in the first leg of their semi-final tie, and speaking after the match, Gallardo explained how tough the recent pre-season has been on the veteran midfielder.
"He told me he was suffering, but he was trying to hide it behind desire and enthusiasm," the coach said.
Aimar ends his career with 83 league appearances for River across two stints - the first coming from 1996-2000.
The 52-time Argentina representative made his name in Europe with Valencia, winning two La Liga crowns and the 2004 Uefa Cup.
Aimar then played with Real Zaragoza and Benfica before a shock move to Johor Darul Takzim in Malaysia in 2013.