I know people will disagree but I don't think he has that natural fitness and endurance to break through to the first team in any major way right now.
Almost every single Foden start has gone the same way in the last year. At the start of the game he looks really good and shows you all the reasons why he gets the hype with ubiquitous levels of game control and skill, but eventually he always becomes uninvolved and isolated.
So I have to assume this constant fizzling out is physical. Whilst he's clearly bigger, he's still relatively small. Plus I don't believe there's a mental problem behind this, I've only ever heard big things about his mentality.
I don't believe he needs a loan or anything to bulk up like I may have used to. Considering the quality he's got and the potential he has I believe it might just be the case that the best option is to be patient.
He's 18 now, which is an age where he/we might think he ought to start. But generally teenagers who feature regularly despite their young age tend to have a real physical element about them, like some were insanely strong (Eg Lukaku, Kompany, Rooney) or fast and agile (Eg Owen, Aguero, Sterling) or insanely fit (Eg Milner). All really talented, but they had that physical element to allow them to start younger.
If we keep Foden around and exposed to the first team and Pep it'll be best for him IMO. Just we all should wait a bit for him to fully fill out before we expect too much. I don't think it'd be a problem if Phil only has limited game time, this year at least. If he can get off the bench a few more times than last year that'd be positive progress enough.