Kaka made his playing return with Milan at the weekend in a 4-0 friendly triumph over Chiasso but admitted he was lacking match fitness.
"To have played one half against a second division Swiss team is simply not enough," AC Milan assistant coach Mauro Tassotti told Gazzetta dello Sport.
"He has only had two training sessions with us and played in a friendly.
"I think he is in good condition.
"We spoke to (Real Madrid coach Carlo) Ancelotti and he guaranteed us that he (Kaka) has always trained well.
"We cannot be in a hurry because in the past few years he has played very little."
The 31-year-old last week admitted he wanted to leave the Bernabeu after four years - having joined from Milan for a reported £56million in 2009.
Milan vice-president Adriano Galliani moved quickly to persuade the 2007 Ballon d'Or winner to return to Italy's top flight.
"We, just like the club fans, are very happy to have Kaka back because besides being a great player he is a wonderful person," Tassotti said. "We hope he will make the difference."
Kaka won La Liga, the Copa del Rey and the Supercopa de Espana during his time at Madrid.