Grealish is looking like a player who's earnestly trying to adapt. He's hugging that touchline, exactly as he's supposed to.
A bit like Foden did, he's going to need a bit of time to adapt to the role and figure out how to get more involved with play, just like Mahrez, Sterling, and Jesus know how to do.
For what it's worth though, once the red card happened and we began to dominate Grealish got VERY involved. If you look at touch/pass numbers he got big, big numbers. So it might just be a confidence thing too; once we had the game in the bag he felt more relaxed about his tactical role.