I don't know, people might hate me for it, but Foden hasn't been setting the world on fire recently. That's normal, all players have warmer and colder streaks, but Grealish has looked more dangerous and confident than ever in his substitute appearances the last three games. I think Grealish also offers better ball-retention and possession control, and with our defensive injury woes, that might be something Pep places a premium on: defend with the ball, and all.
You're probably right that Foden will still be preferred, but I wouldn't be surprised if Grealish got the nod.