Are United possibly transitioning to a 3-5-1-1? I've seen that you've played it in most of the pre-season games, but wasn't sure if this was just down to available personnel (lack of attackers).
If so - then yeah, they really need much greater creativity from the wide areas. I think the right-hand side could be OK, you have Valencia and Dalot as options, the latter looks a really good prospect but may take a while to develop, and if Valencia isn't cutting it from a crossing perspective, you could move Young over.
But LWB really needs addressing. Young is a decent stop-gap, he's worked hard to develop the defensive side of his game similar to Valencia and is a better crosser, but being naturally right-footed makes him easier to defend against, you know he's going to cut inside and when you're aiming in-swinging crosses, really the only place you're going to put it to create any danger is the far post and teams know that and can defend that zone.
I think you need a natural left-footer there. Whether that's Sandro who would be ideal, or Alex Telles instead, somebody good who can get to the byline and put balls in to the box and own that left-hand side.