You are jumping to conclusions on the basis of one game, and that game was at Anfield. If you had to judge Aguero, Kompany or Fernandinho on the basis of their performances at Anfield, I'd dread the conclusion you'd arrive at.
The assumptions you make are dubious. You are conflating your own interpretations of Pep's reasons to play Grealish at false 9 with his real reasons. Chelsea and Liverpool are utterly different teams. Foden did a great job at Chelsea (judge his performance relative to the team performance, not relative to individual stats), we were absolutely fantastic there!, but Pep decided that against Liverpool/Milner he would be more useful on the left. Too early to tell whether Pep will continue to play Foden on the left and whether Grealish can(not) excel in midfield or as a false 9.
It's Pep Guardiola who determines what our best XI is, not guys on the internet. As long as Grealish features in the starting XI in the biggest games and we play well, he's part of our best XI. It's as simple as that. To think about our players in terms of a priori fixed roles (e.g. false 9 or left winger) is something many posters here do, but not the best manager in the world. Relax and leave it to him to decide how to use both Foden and Grealish, he's likely to come with good solutions.