Rivaldo fired Ailton after the club lost 2-0 to Portuguesa on Wednesday night, their third defeat in five games.
"It is not easy to put together a team quickly for a championship as tough as the Paulista," the former Brazil international said in a post on the photo-sharing app.
"And I'd like to say that it wasn't his fault, but when the results don't come we need changes to get the players going again and as president that was the decision I had to take after the game at the Caninde (stadium)."
Some supporters were unhappy at how the news was announced, which led the former world player of the year to clarify an hour later that Ailton had been informed after Wednesday's match.
"Ailton was let go in the dressing room after the game in front of the whole team, I came here to communicate the decision to my followers," he said.
Rivaldo is still turning out for Mogi Mirim aged 41 and last week played alongside his son in the team's 1-0 loss to Ituano.
- Sports & Recreation