AC Milan, Pioli eyes three-man defence; centre-back in January
According to Sky Italia (via MilanNews.it), the feeling is that Milan are moving towards having three-man at the back, allowing Theo Hernandez to play in a more advanced role. However, as of now, they don't really have the squad for it.
Therefore, as Sky continues, the plan is to sign a centre-back in January to fill that void. Furthermore, Mattia Caldara will soon be back to full fitness, which will give the Rossoneri options at the back. Then again, it remains to be seen how Pioli will deploy the system.
Currently, it's three-man backline when in position and a normal four-man backline when they defend. If a defender arrives, though, Pioli might decide to field a proper formation that is more suited for the system.