I think that something happened around the time between away games in Copenhagen and Liverpool.
-- Quote from Pep's post match interview after 0-0 draw with Copenhagen--
Pep Guardiola: "Many players didn't start because they were exhausted, tired, fatigued, niggles. Erling [Haaland] didn't feel really good after Southampton, Phil [Foden] had some disturbing problems, Bernardo [Silva] told us yesterday he was so tired."
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Afterwards Foden played Liverpool game. Then his minutes are reduced from playing almost all the games at the beginning of the season to rotating with the others. Pep's words implies that Foden carries some discomfort. Maybe an injury. Maybe he overplays at the beginning of the season.