Portugal international Ronaldo joined Juve from Real Madrid in a high-profile €100million transfer ahead of the 2018-19 campaign on a lucrative four-year contract.
He is now into the final year of that deal and has been linked with a move away from Turin after requesting to be named among the substitutes against Udinese on Sunday.
Gigli previously questioned why incumbent president Andrea Agnelli sanctioned the signing of Ronaldo and has once again claimed Juve made a big error when bringing in the superstar forward.
"I've always been honest: signing Ronaldo was a mistake," Gigli, who was in charge between 2006 and 2009, told SerieANews.com. "It's impossible to recoup the investment and it will remain that way.
"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 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."
Paris Saint-Germain had been tipped to move for Ronaldo prior to signing Lionel Messi earlier this month, while a move to Major League Soccer has also been touted.
However, head coach Allegri recently insisted he has been assured by Ronaldo that he does not intend to go anywhere this month.
With the transfer window set to close in a week's time, Gigli has urged Juve to focus on finding a suitor for Ronaldo while also reaching an agreement with Paulo Dybala over a new deal.
"Renewing Dybala and selling Ronaldo in the next week would be best for all parties," he said. "But I don't believe someone like Ronaldo can be put on the market in seven days."
Ronaldo has scored 101 goals in 134 appearances since joining Juventus and won five trophies across his first three seasons in Italy.
The Portuguese netted 29 goals in 33 league games last term to top the scoring charts, becoming the first player to win the Golden Boot in Serie A, LaLiga and the Premier League.
He scored two more the second before, however, which was a 10-goal increase on his first campaign in Italy.