Football is a dynamic game and narratives change every day. You really need to move on from "Sterling's been shite all season, so Foden deserves to start". As an aside, this is why so many managers are absolutely fucking awful - they put too much weight on what happened in the past few months/years and not enough on the immediate past/present.
"Every match and training session, you start from zero" is cliche at this point, but it is 100% true. If Foden performs good in one match, but is not better in training the next day than the players he's competing against, he won't start. Only Pep and the coaching staff have access to that information, so you have to trust them as long as their decisions deliver good results/performances (and they are).
One of the things that people should do before criticising the manager is ask "what could be the reasons for him doing that" instead of "ffs why hasn't he done what I want". More often that not, you will find the answers yourself. All you have to do is ask yourself the question, instead of taking the immediate and easy route of whinging and moaning at every turn.