I honestly don't know what to think about Martial;
He is quite young , relatively inexperienced in a dysfunctional squad, probably should be starting infront of Sanchez, 1st 1/2 of the season was doing brilliantly alternating with MR on the left hand side, was connecting reasonably well with Lukaku as well, had a shockingly bad left back behind him which didn't make attacking moves any easier
He drifted out of games pretty badly and too often, quite a few games he was completely invisible (and the "Elite" level can be harsh in this respect), given the 18 months - 2 years difference between Martial and Rashford, I would prefer to keep Rashford as he is locally born and within that time frame can easily be better than Martial (of course it could easily go the other way too), left hand side of the team is over loaded and unfortunately we will never sell Sanchez (even though after what he has shown vs the minutes he has been given, he should be 1st out the door) so Martial is the best one to sell to be able to strengthen the right hand side - which Man Utd are desperately weak on
(sub topic - but wondering in a few games at start of the season whether JM will risk playing Dalot as a right winger for his speed/ crossing ability infront of Valencia until the 19 yo is a little more experienced at this level?)
Even with Fred joining (when its eventually made official) and therefore allowing Pogba further upfield - if this is what happens - Im not entirely convinced Sanchez will be much better than he already has been. I know he is capable of it, but so was Mata theoretically, but he has been relatively dire after moving from Chelsea (or rather has missed loads of very easy goals , that were harder to miss than score - yet that's what he did over and over again in the last 3 or so years).
I strongly believe Utd still need one full back (that's totally ready to go as a starter), a midfielder / box to box type (to cover / alternate for Fred and Matic at very least) - a centre half (and would happily sell at least two of Jones, Smalling and Rojo) and a goal scorer (whether that's a striker or a right winger I don't mind which)
.
He reminds of a Salah type player who was also written off by Jose whilst at Chelsea, give him a bit of freedom and the difference is immeasurable.
Not to mention a lot of time in a different league to "grow up" and become more experienced