According to what is reported by Tuttosport, Stefano Pioli has strongly focused on the 4-3-3 formation. Despite achieving important and great results with the 4-2-3-1, it is unlikely to change course in the coming weeks. There are primarily three reasons for this:
the key players in the double midfield from last season are no longer there - Tonali has been sold, Bennacer is out until February -; the new arrivals are training in the 4-3-3 formation; the pure attacking midfielders have been "banished" (Brahim Diaz has returned to Real Madrid, De Ketelaere and Adli have had limited appearances in tests and are on the market). Therefore, they will proceed with the 4-3-3 formation, although during the season it will be possible to see the old system in the championship version, with a midfielder in the attacking "number 10" role, as Kessie or Krunic himself were.
Due to squad dynamics and Pioli's understanding of the game, Krunic is currently the central pivot in the three-man midfield: Loftus-Cheek is a box-to-box midfielder, Reijnders has playmaking and insertion abilities that fit well with the left midfield role, and when Musah arrives, he will likely be an alternative for the two central midfield roles or a candidate for the defensive midfielder role in the 4-2-3-1 formation. Considering that Pobega has his roots as a central midfielder and found little space in the 4-2-3-1 formation last season, it's clear that Pioli lacks a true alternative to Krunic. Considering the highly offensive trio and the insertion skills of Loftus-Cheek, Reijnders, and Musah, the player who will complete the midfield department needs to be disciplined and proficient in interceptions. A type of player that Milan has always had in its successful seasons, think of Rijkaard, Desailly, Pirlo himself in the deep-lying playmaker role, and his successor, a player that Pioli would always field if available today: Van Bommel.
AC Milan is naturally considering various profiles, two from Bologna: Nicolàs Dominguez - who, by the way, is also being pursued by Turkish clubs who would "release" him if they secure Krunic - and Jerdy Schouten. The former is more of a playmaker, the latter a defensive pivot. Among the names on the list is also Ibrahima Sissoko from Strasbourg.