There is no chance a manager can implement a system to get the best out of every single player in his squad, especially when 90% of them are players he didn't even purchase himself. Jose Mourinho never was and never will be a posession manager. His tactics have always been about quick transitions and movement, both things Martial needs to improve on.
Left forward may well be a good position for Martial, but we lack width as it is, losing out on the current widemen we have is going to make things even worse. It's not just the manager who has to adapt to the players, the players need to adapt to the manager too.
Also this talk about him needing to play as a forward is rubbish imo, just excuses to explain his disappointing form. His best asset is his ability on the ball, which is one of the most important attributes for a wide player. His problem hasn't been his position, it has been his slow and care-free playstyle, lack of movement etc. Whether this is down to tactics or faults of his own, I'm not sure but given Jose obviously doesn't prefer him currently, it's fair to say he's not doing what he expects.
Lastly just because Jose thinks the squad needs a proper winger (which we do, we don't have a single one), doesn't mean he doesn't see Martial in his plans. If he wasn't in his plans he'd be gone, we'd hardly be short on suitors.