a lot of our young players cant quite break in to the first team because of their physique, you can be dominating at under 18 level, but break in to the first team and no matter how talented you are you will be bullied off the ball. Diaz is another one. Of course we do have rare exeptions, look at Spanish dave and there have been others.
look how Chelsea dominate at under 18 level with their physically strong under 18's that look 24 !
I don't know why the coaches cant see this or do they prefer to just take it very slowly, Foden will fill out by the time he is 22 or 23 but why for now don't they change their training regime to have less cardio and more short strength stamina boosts. Foden must have a fast metabolism, so 6 mars bars, fish and chips and a can of coke in one sitting wont put weight on if he is running round like mad on the training field. I do think they don't get the balance right and at this rate we wont get to see foden playing regular until he puts almost a stone on by the time he is 21.
Within 6 months with correct diet and training you could have half a stone on the guy and he would be pushing for a regular first team place.