Antonio Nocerino rushed through a deadline-beating transfer from Palermo to Milan.
The Italy midfielder had already been linked last week after Mathieu Flamini was ruled out for five to six months by a knee injury.
The Rossoneri repeatedly assured their market was completed for the summer, but Nocerino has been added to the squad minutes before the deadline.
Milan have bought Nocerino on a permanent basis in exchange for an undisclosed fee, thought to be just €0.5m, plus co-ownership of youngster Ricardo Ferreira.
The player signed a five-year contract that was registered with the Lega Calcio just minutes before the 18.00 UK time deadline.
Nocerino would've been out of contract with Palermo in June.