Kaka has had a disappointing spell with Madrid since leaving Milan in 2009 and is believed to be ready to leave the Spanish capital in January.
The Brazil international spent six seasons at Milan, winning a league title and the Champions League, and Allegri would welcome the chance to add Kaka to his squad.
“It’s not yet definite that Kaka will come and so we have to wait for that," Allegri said. "If he comes, the technical level of the team rises without a doubt."
In a separate interview, he added: "It is difficult to speculate on the percentage for Kaka, but as a coach I look at this issue with great interest."
Allegri was also cautious about the club's reported interest in City striker Mario Balotelli, who has been repeatedly linked with the Rossoneri over the past 12 months.
"Balotelli is a Manchester City player, and I have not spoken with Roberto Mancini about him, and I think he still has continuing and important qualities," said Allegri.