It's a question of numbers with Welbeck.
Last season, DW scored about a goal every 4 shots, which is roughly the same as RVP and slightly better than Rooney. The problem is that he only averaged 1.8 shots per game, whilst the other two averaged 3 and 3.5 respectively. What this meant was that Rooney and RVP would score a goal every week, so all their bad misses would be easily forgotten. Meanwhile, Welbeck wasn't playing regularly so he might have ended a scoring run on some given Saturday, been rested the next match, had 2 missed shots the game after that, then been put on the bench for two weeks before missing a shot in the following match and only finally scoring again after another week's break. This would have meant he'd still be matching RVP's scoring ratio of 1 in 5 shots, but the misses and the time in between goals would have entrenched opinions and made him the butt of jokes.
His problem isn't his finishing, it's the fact he makes the wrong kind of runs so he doesn't get on the end of chances. This doesn't mean his runs aren't still useful for his teammates, mind you. It's just that they detract from his own statistics.
If LvG gives him matches up front and forces him to stay around the box, he'll score plenty of goals this season.