I think Martial can be our goalscoring winger. He got 17 goals under Van Gaal and I think his goals per game ratio wasn't so bad last season. If Mourinho can give him the starts next season, he can replicate that.
His ratio was awful .... in the 1 goal per 350+mins range in the PL (I've posted this once, can't be arsed looking for it again) and he got plenty of minutes last season (not quite as many as Rashford but not too far off). It's amazing that in this thread most people seem to have completely binned Rashford and Martial, both enthusiastically espoused here on RC as new world class superstars. Martial scores when he's played at CF, he's not a winger.
However, SirMattBusby above poses the right question, how is it everyone is writting off some of your current squad ? And has Mourinho done the same ?
You need to look at why Martial crashed and burned last season. He's much better than that but it seems Mourinho, as his history proves conclusively, is no lover of giving sufficient time to develop young talent, and if you end up buying half of the players put forward in this thread then I forecast Martial will be gone Summer 2018.
Rashford is another one, yes you're going to have plenty of games but he is another one with a very poor goal return in the PL, again 1 goal per 300+ mins (even Origi had almost a 50% better return by ratio of goals to mins and so far he's not in Rashford's class). Lingard is surely gone 2018 too as where is he going to get serious game time next season ? The LC and FAC won't do it. So does Mourinho have the patience to persist with them ?
Personally I see United as requiring players in nearly the same positions as Liverpool : LB, CB, CM, Striker. Four top class players (though not sure Keane falls into that bracket as he seems like he'll be your new CB) and it wouldn't mean the end of Martial & Rashford, though they would be pushed out onto the wings, or playing off the CF though I don't see that happening.
If you don't, however, see them as wide midfielders then they will see little action, they didn't get as many minutes as they would have liked last season, with new players in the squad their opportunities will only decrease, and that won't go down well with them in a WC year.
You have to consider, as one of the posters made the point above, at whether the lack of goals is due to the players or the manager (formation & tactics). The manager isn't going to change, Mourinho being set in his ways and his title winning Chelsea team is now his PL blueprint, he rarely plays with two strikers on the pitch, and why are people calling for another AM when you already have Pogba, Mkhitaryan and Mata ? Pogba will clearly play as the most advanced midfielder, effectively as AM, next season and that leaves a striker and two wide players.
So instead of playing FM and concentrating all your attention on bringing in a whole host attacking players (LW, AM, 2 strikers), as posters have mentioned in this thread, you need to think like Mourinho, because a) that isn't him and b) I'd be shocked if he's suddenly going to suddenly bin all of his attacking players and start next season with virtually a whole new attack. As I said LB, CB, CM & one Striker, that's what I think you'll get.