Against a side like Chelsea or Liverpool and teams that high press I feel Foden is great as the false 9. When playing out of the back, he can drop deep, receive the ball in the midfield and either lay it off or make one of his mazing runs like a 10 which helps advance the ball up the pitch. See the Gladbach game last season where he laid it off to Gundogan to score. I was in favor of getting Grealish for the very reason he could go on the left and Phil in the middle in these types of games.
We may lose a little bit by not having an actual striker in the middle when we do get in the attacking half, but I think Pep figures he wants the ball first. Put pressure on the other team and make them defend. We don't want to be defending in our own half for much of the game.
Phil's pace and energy for pressing and counter pressing is impressive also.