Getting the best out of underperforming players

Pogba has been mixed for United this season, and given the rumoured exchanges between the two, it would be another PR boon if he can improve him.
Sanchez has done poorly since he joined the club, certainly vis a vis his Arsenal form. His goal and winner vs. Newcastle United could be a new dawn for him. This remains to be seen though, but a vastly enhanced Alexis could be another plus for Mourinho.
It's even been reported that Martial says there is no rift between him and Mourinho, and he is happy at the club.
Martial and Rashford would be similar to Pogba and Alexis, though they need to be utilised more thoroughly. Rashford in particular, after his initial brace vs. Arsenal in 2016, has not progressed nor fulfilled his early promise.
Part of United's woes this season have been underperforming players, and if United is to improve, they need to get the best from them.