Longstaff is more an all-action, two-way English mid, which means he's not likely slide-rule any passes to the players upfront. He can ping passes and he can tackle guys, and he can hold up the ball.
He's probably seen mostly as an eight or a six, but I doubt he'd be used as the false nine. Maybe. Early days.
Rosted is Long's natural backup as he's okay in the air and we need the central guy to win aerial balls. Not sure he'd rate Mabika highly enough to displace him.