Mentality is a bit subjective. I think the most common criteria is sprinting frankly. Beyond that, fans know little.
A top player, one with genuinely special ability, should hopefully be determined to be a great player. The notion that players ‘really want to play for Manchester United’ is conveniently often aligned with players who can’t get into a better team than Manchester United. For a British player, if he was not a United fan, he’s more likely to have grown up hating us than anything else. The thing is, a player should be ambitious and want to go to the top, so if he played for Swansea, of course he should want to go to United. But that also depends on his options. Ramsey and Bale, when presented with an alternative superclub at the time, both went elsewhere. Choosing us over a move to Leeds or Newcastle proves nothing.
A player like Neymar is probably seen as uncommitted, in fact, I’ve read his name in this very thread as an example of a player to avoid. However, he is extremely committed to being a great player, and as a result never lets his team down for club or country. He can never be accused of ‘not playing’. He is protective enough of his own reputation for that. A club like Real or Barca would never say ‘we should stay away from a player like Neymar’. They would sign him, he will be great there, and win lots of trophies. This has nothing to do with any love or commitment to them. Neymar would leave Barcelona and sign for Madrid today. The point is, they match his ambitions.
When you aren’t a very good team it’s easy to view every top player as one to avoid. The likes of Neymar would be as happy to play for us as anyone else, if we were at the top of the game. Players who don’t have that option are happy to be here than at small and average clubs. It’s normal. There is little to suggest that Pogba hates United, as is often implied. He’s not even Latin and just doesn’t like the city or something. He considers it home. I’m sure if we were a stronger side he’d have no doubts. That problem will remain unless we just adopt a strategy of signing players who wouldn’t be wanted at better teams.