Midfielders: Marco Verratti, Vitinha, Fabian Ruiz
Assuming Galtier uses a three-man D-line along with two wing-backs, then it'd make little sense to use Danilo (as Marquinhos will man the CB/DM role himself). That leaves the three midfield players to pick themselves nearly automatically.
Marco Verratti should start in the Regista role ahead of Marquinhos in the deepest midfield position. Vitinha should be the one linking Verratti and the defensive line with the offensive block of the team given he's been atrocious at the no. 10 position and further places.
All of the above leaves one lone "advanced" position higher up the pitch while still in the midfield. Pencil in Fabian Ruiz unless Galtier goes a little bit more offensive and chooses Carlos Soler, or a little bit more conservative and picks Warren Zaire-Emery. Not seeing that happening, though.