Attitudes aren't set in stone, in my opinion. Or at least, not in the case of all players (a firm exception: I'd never want Neymar at my club, because I genuinely believe that he'll be trouble virtually wherever he goes).
But in many cases a player's approach and attitude can be moulded. And if there's one thing I trust Pep on, it's his ability to do that. This lad's 25, I honestly know virtually nothing about him, but all the reports are good as far as ability goes. For the rest, well, if he comes, and he's putting a shift in, I'll get behind him.
To be fair on Mendy. I sometimes wonder whether his high jinks and occasional silliness aren't the expression of an immense frustration at seeing this wonderful team do all these wonderful things, and thinking “I could be a part of that, but my body's betraying me".