This argument rests on the assumption that you can only judge a player in real first team games (as opposed to training). I don't have an opinion on that particular argument. That said, if you believe that someone like Pep can make a judgement on a player based on training (and everything else at his disposal), I expect Roberts will get his chance...it just may not be in the first team.
I don't believe Pep, or anyone else, can decide from that, at all. I think it's just something people say, on here.
They could tell if a player was total shit, or stupid, or dead slow, or a coward, but if Roberts was any of those things he wouldn't have been playing in cup finals.
We have a load of talented kids, many of whom will hold their own in training, by comparison to one or two of the experienced ones, who probably don't train that well.
The only time you find out about any, is when they are playing in a proper game. It's taken Otamendi two seasons so far & he's not there. Stones hasn't got it after a season. Kolarov will never have it. Fernando. Delph. There are plenty we signed under Mancini & Pellegrini, who played often enough, but never got it.
If anything, it's been proven without a shadow of a doubt, that managers can't just look at players on a training pitch & make a judgement.