Ronaldo has just one year left on the four-year deal he signed when he joined Juventus from Real Madrid for €100m in the summer of 2018.
The Portugal superstar has been linked with a move to Manchester City and Paris Saint Germain in recent days.
Ronaldo reportedly asked not to be involved from the start in Juventus’ Serie A opener against Udinese over the weekend as he tries to negotiate an exit from the Allianz Stadium.
Gigli believes Juventus will be better off without Ronaldo in the side and has asked for the former Manchester United forward to be sold.
"I've always been honest: signing Ronaldo was a mistake. It's impossible to recoup the investment and it will remain that way,” Gigli told SerieANews.
"He's a great player, but I have to be honest – the sooner he leaves, the better for him and for Juventus.
"I hope that [Juve head coach] Massimiliano Allegri knows how to continue using him like he did against Udinese. That is, with intelligence and when the game is in progress.
"Ronaldo hampers Juventus' attack. Without him, they can do excellent things in collective terms."
Ronaldo has won two Serie A titles with Juventus and scored over 100 goals for the Old Lady.
