Fans tend to be defensive at times yeah, but pretty much everyone in here has a go at their own players when they're playing shit. And every football fan in general will give out if a player they don't like is starting in the team as we all love to play manager.
The most valuable opinion will generally be the well-informed neutral. Fans tend to be defensive of their own players in general non-club specific environments like this.
I'd always take a fans opinion on a player over a neutrals plainly because they see more of them and most fans are realistic about their players.
Thing about Man U is, they get far more coverage than other teams, so the average neutral will have seen a lot more of Welbeck and a good deal of full matches compared to someone like Ricky Lambert, who I can't honestly say I have much of an opinion of.
Neutral fans tend to be overly critical of big teams players as well. We all love to see a player struggling at rival clubs.