Jack Doohan, 22, was kicked out of his team Alpine in May after just six races.
The Australian was replaced by Argentine Franco Colapinto, who finished 16th in the Emilia Romagna GP in Italy on Sunday, after a serious crash in qualifying.
"Very impressive", it says in the allegedly fabricated comment attributed to Mick Doohan, with a picture of the crash.
"It was fabricated by Argentine fans who are trying to portray me and my family in a negative light", writes Jack Doohan himself on Instagram.
"Please, stop harassing my family", he continues about the image that got some spread in Argentine media.
Alpine also condemns the incident.
"We encourage everyone to remember that behind the visor of these superhuman athletes is a person, a person with feelings, family, friends and loved ones", writes the team.
