Throughout his entire career Jose has preffered known quantities in terms of players that he knows will give him solid performances, week in week out, and are in their prime. Sanchez is the prime example.
Manchester United was never the right fit for him. Rashford, Martial, Pogba. Arguable our best players, but too much flair and inconsistency for Mourinho. The error here isn't Mourinho's judgement, at the time both Rashford and Martial's consistency wasn't there. The error was the appointment of Mourinho in the first place.
And of course they can play together, talented players should always be on the pitch. It is the managers job to mould the team, Mourinho just didn't want to try.