In Lukaku, Sanchez, Martial, Lingard, and Rashford we have 5 forwards for 3 positions. (I haven't included Mata as his contribution has mostly been as a midfielder this season, and he's no more a forward than Pogba)
What other club our size would be expected to go into the season with fewer options?
Two of the players are homegrown, and apart from Sanchez, they are all still developing.
Mourinho has said that he's not looking to sign another forward player in the summer.
It's up to Rashford and Martial to perform so well that they make the LF/CF position their own... if they can't displace Sanchez or Lukaku, they will either need to learn to play on the right (I know... development, how horrible) or become an impact sub.
We are probably losing Carrick, Fellaini, Darmian, Blind, Borthwick-Jackson, Shaw... and players like Jones, Smalling, Rojo etc must have question marks by their names. Our focus is going to be on defence and midfield this summer.
The fact that Martial has 2 years left on his contract probably just means that he wants to be in a better negotiating position in a years' time. He wants more money and more playing time... big surprise.
Edit:
I'd like to see what Martial could do on the right if we had 3 in midfield. Take away his extra defensive responsibilities and tell him to tear the opposition a new one on that side... a little confidence would go a long way.