Real Madrid boss Jose Mourinho has suggested Kaka is unlikely to ever regain the form he showed at AC Milan.
• Blog: Kaka's last chance?
• Galliani: Kaka deal wasn't close
• Jose issues warning to stars
Kaka, 30, was named the FIFA World Player of the Year in 2007 after helping Milan to glory in the Champions League, and Madrid broke the world transfer record when taking him to the Bernabeu in June 2009.
However, he has struggled to live up to expectations in Spain, with a series of injuries hampering his chances.
"What has happened to Kaka has happened before to many great players - stars who, for one reason or another, were not able to maintain the same level at new clubs," he told AS. "Obviously football is not mathematics and players are not robots. Kaka arrived with physical issues and did not play regularly in his first two seasons. Last year he had very, very good periods and did good things for us."
The Brazilian would have limited chances to feature this season, Mourinho said.
"His situation is not easy as we have other options, but he has accepted the challenge," he said. "He is training well. When we need to use him we will use him and try to get the best out of him. When that moment comes, Kaka will respond and help Madrid."
In contrast, Luka Modric, this summer's €37 million signing from Tottenham, is set to be a key player for Madrid this season.
"He was the only signing we thought about, and we did it because I like him as a player," Mourinho said. "I was sure he will fit in well in the dressing room and because Madrid's supporters would like him.
"He has talent and tactical discipline in equal amounts. He is a team player who looks for and makes himself available for team-mates. He also has a touch of class, or magic, that the Bernabeu likes. We have signed Modric as he will make us enjoy ourselves and help us to win."