Fabio Capello described Ronaldo as the most gifted talent he ever coached but said no other player caused him more problems.
Ronaldo played under the Italian coach for six months at Real Madrid before being sold to Milan in January 2007, having scored 104 goals in 177 appearances for Los Blancos.
The Brazil icon made just seven LaLiga appearances, scoring once, in that campaign after finding himself out of the team due to the arrival of Ruud van Nistelrooy and arguments over his weight.
Capello, speaking to Sky Sport Italia, had no doubts about Ronaldo's natural gifts but said his partying antics were a cause of consternation.
"The greatest talent I ever coached was Ronaldo," he said.
"But at the same time he was the player who created the most problems for me in the dressing room.
"He used to throw parties and do everything. Once Ruud van Nistelrooy said to me 'coach, the dressing room smells like alcohol'.
"Then Ronaldo went to Milan and we started to win, but if we're talking about talent then he was the greatest, without a doubt."
Madrid went on to win LaLiga under Capello in that campaign, while Ronaldo was released by Milan at the end of the 2007-08 season – during which he sustained a serious knee injury.