Don't want Mac Allister at all. He's not bad, he's just nothing like what we need. We need cover for Casemiro who is young and can be eventual replacement (as in a young DM who has good spatial awareness, good positioning, and is talented... So Lavia). And then we need a partner for Casemiro who is a deep playmaker, someone who can receive the ball under pressure on the half turn, who is press resistant, and can make line breaking passes, can control the tempo of games. There aren't many of them, but to be an elite side, it is what you need.
Mac Allister would be along the lines of Fred and Sabitzer. They aren't the ideal fits for us, but good squad players. Unless all of Fred, McTominay and sabitzer are leaving, we just don't need more squad players in midfield apart from a Casemiro backup.