Cassano believes that the Portuguese captain is not playing at the level he was at when he was competing for trophies and honors.
The Red Devils attacker has made eight appearances this season, scoring just one goal.
He is coming off the back of a disappointing performance in the UEFA Nations League with the Portuguese national team, who lost 1-0 to Spain on September 27.
There have been calls for Fernando Santos to drop the legendary forward to the substitutes bench ahead of the 2022 FIFA World Cup, which kicks off in November.
Cassano said that Cristiano Ronaldo should consider retirement amidst his difficult start to the campaign:
"At this point, Cristiano should do himself a favour and understand that if you don't reach a certain level, you should close the shop. He won everything, he was a phenomenon. Stop! You have to stop or you'll only run into the bench."
Cristiano Ronaldo has carved out a phenomenal career since making his debut for Sporting CP in 2002.
He has won five Ballon's d'Or, five UEFA Champions League trophies, the UEFA European Championship and three Premier League titles, among other honors.
However, the pressure is now mounting on the veteran striker to find form for both club and country,
Manchester United manager Erik ten Hag dropped the Portuguese following his performance in the side's demoralizing 4-0 defeat to Brentford on August 13.
The Red Devils won four games on the bounce following that decision, with many believing he should not be starting for the Premier League club.
Despite this, Ronaldo vowed to carry on playing into 2024 with Portugal at the European Championships this past week.
Can Manchester United forward Cristiano Ronaldo bounce back?
It is disappointing to see Cristiano Ronaldo struggle as much as he has this season.
The legendary forward has been for many one of the greatest players to ever grace the game, but his regression has now started to occur.
The problem Ronaldo has is that his professionalism sees him wanting to play regular roles for both club and country.
The former Real Madrid forward is not one to entertain the idea of sitting on the bench and watching his teammates from afar.
However, that might be the way to go for Ronaldo when it comes to his role at Manchester United for now.
A move for the Portuguese may be for the best come next summer or even in the upcoming January transfer window to get back to playing regularly.
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