Sancho and Greenwood's positions are mutually exclusive. Both will benefit from each other. Sacho will be able to focus on RW whilst Greenwood will have free roaming license. We will see multiple combinations of Sancho, Rashford, Mason and Martial, giving enough rest to all of them, at the same time not dropping the standard of the team till 90 mins even after substitution.
Same with Bruno, Pogba and new AM (if we get him). There too we will see multiple combinations depending on the opposition, situation. No burning out entire midfield, at the same time keeping the standard even after substituion.
Similar situation with Matic, Fred and McT. Again, standard will be maintained till the final whistle.