AC Milan have suffered an injury blow with French midfielder Mathieu Flamini set to be ruled out for at least five months.
Flamini, 27, hurt his right knee during last weekend's 2-1 win over Juventus in the Berlusconi Trophy, and further tests have confirmed ligament damage. He will have to undergo knee surgery and is expected to be sidelined until next year.
Flamini, who made 22 league appearances to help AC Milan win the Scudetto last season, will miss the first half of the season, including the group stages of the Champions League.
Aquilani joined on loan from Liverpool until the end of the season and Milan have an option to make the 27-year-old's stay permanent if Aquilani makes at least 25 league appearances for the Rossoneri this season.
"I believe that with Aquilani's arrival our transfer market is complete," Allegri said. "He can cover all roles in midfield and he is a player that has great quality with attacking characteristics. I believe he will adapt well in our midfield line."
Aquilani spent last season on loan at Juventus and is under contract with Liverpool until June 2014.