Roberts was a known 'hot property' at Fulham & City stepped in ahead of, well just about everybody & bought him for decent money.
Foden was here last season & both him & Diaz were involved with the first team, so Pep knows them better. If the 3 of them went to Celtic today, I'm not sure those two would necessarily outperform him there, in fact my money would be on Roberts in the short term, he has more end product for senior football.
But they are younger, & he hasn't spent a year with Guardiola like they have, he's spent it with Brendan Rodgers.
If we decide we donn't want him, we should sell him. If & when he was to come back, there will be even more players in front of him.