I think people are missing an important aspect wrt Young's game and his failed crosses.
Whilst I like everyone else wish that he were better at picking out a player when he gets the chance, the defence still have to cover Young (and those in the box) as if he might. He wears them down, he provides a consistent focus drawing resources towards him and away from others, perhaps especially Blind. This gives Blind greater opportunities, and their synergy in both attack and defence gives Blind greater freedom. Between the two of them (and Fellaini) they control that side, give out-balls and avenues for forward progress, their ball retention until Young tries to cross is excellent. In combination with the trio on the other wing they cause major headaches for our opponents.
Now if LVG can work out a way to improve Ashley's final ball...
This is the main reason that Young made most of the poorer opposition concerned about him (even as a left wing back).
I feel like I'm watching Young at Aston Villa again. Back then, he'd have been lauded for such performances with Aston Villa with him beating players, causing the opposition problems with his pace, and crossing balls into dangerous areas without them making it to the players. With us, however, given that we've had better wingers in our times, we expect more than what Young's been showing at the moment. It's not enough for Young to just play the ball into dangerous areas and not aim for the players. In Aston Villa, he got away with it because O'Neill flooded the opponent's box with the strikers, the right winger, and a midfielder, not to mention the large number of set pieces he took and delivered into the box with big, tall players like Laursen, Knight, Harewood, Carew, etc., to aim for. I should also mention that O'Neill did play a fast-paced, counterattacking game as well with Villa, which suited Young and co. very well.
Man. Utd. played and still play differently from O'Neill's Aston Villa, so Young isn't going to create enough goalscoring opportunities playing in his usual way. We don't flood the box too often, and until we had Fellaini playing well, we didn't have a big, strong player who can challenge for crosses into the box. This is why, even if he's been back to his best, we still don't see him really create enough chances as he rarely looks up and picks players. Given how di Maria hasn't been the most efficient on the ball, he still spots players and seeks to play them in even if it doesn't always work. Young, however, does none of that. Even Valencia looks up more often than Young and puts in better deliveries, overall, than Young does.
However, what makes Young so dangerous is the fact that he plays balls into dangerous areas. This keeps the opposition defenders on their toes and makes them sink deeper into the box, leaving space in the box for players like Herrera, Rooney, and Mata to enter and score goals from. Of course, if he aimed his crosses rather than play them in blindly, he'd be one of the league's best wingers. What keeps him ahead of di Maria and Januzaj, at the moment, is his positional discipline, his ability to beat players more often than those two, his defensive discipline, and his good combinations with his full back. Both di Maria and Januzaj haven't contributed enough defensively for some time, both haven't been as consistent as Young in beating their man, and both don't combine that well with their full backs at the moment.
Still, though, this cannot mean that he will do well doing forward against decent defenders. Against Newcastle away, Janmaat dealt well with Young, and Young was effectively ineffective. Against Liverpool, Young had a relatively quiet game when compared to his performance against Spurs, where he didn't even create a chance (out of my own memory).
Against Man. City, I see Young being quiet again, going forward, but his link-ups with Fellaini and Blind will make them more dangerous than if di Maria or Januzaj played on the left wing. 2 dangerous players over 1 dangerous player is a no-brainer, at the moment.
TL;DR: Young's Aston Villa style won't be enough for him to create chances here; his other qualities make Blind and Fellaini more dangerous, though. di Maria and Januzaj might not be able to make our left side more dangerous than Young has at this moment.