Mexico's America name Ramirez as coach
The club confirmed in a statement that Ramirez, who coached the Mexican team that won the under-17 world championship in 2005, would replace Diaz.
Ramirez coached the senior Mexican side on a caretaker basis for three months last year and he will leave his job as head of Mexico's national youth teams to take up the post.
America dominated Mexican football during the 1980s and are one of the country's richest clubs but have lost their grip on the game in the last few years.
The last year has seen a flurry of coaches come and go as the club attempted to re-establish themselves as a major power.
Argentine Daniel Brailovsky was fired last February, having taken over only four months earlier. He was replaced by compatriot Ruben Omar Romano, who lasted only 10 games.
Juan Antonio Luna briefly took charge before the job was handed to Diaz, another Argentine.
America have spent around $20 million on new players during the mid-season break but have made an indifferent start to the Clausura championship, winning only one of their first four matches.