Of course his current form of goalscoring is unsustainable, they've all been screamers. Runs like that an incredibly efficient finishing are never sustainable. Signs that are good that point towards possibly sustaining runs of scoring is if you constantly get in good chances, even if you miss them. Allabout the 'xg' pretty much.
Aside from the goals though, Lingards game has definitely improved loads anyway. When the goals dry up, it won't mean that has gone back to old form, as he looks like a very good player these days. Takes great touches, always attacks the space, links up well, finds good pockets of space to receive passes, and obviously his pressing off the ball.
If Sanchez does come in, I'm sure he'll still have a similar role. The way I see it, is Theres 6 spots between the midfield and attack. Pogba, matic and lukaku start when fit. Sanchez would start when fit as well (and perhaps throw lukaku into less of a guaranteed start when fit). The other 2 spots will normally be part of the 3 behind lukaku, and so 2 of Mata, martial, rashford and lingard, whoever is in better form (like it's been this season).