Every time Bruno talks about him he caveats it with essentially "sometimes he is a twat but he's working harder now"
I think there's a genuine concern from the players and the staff that he was a bit of a dickhead when he first came into the first team, and still is from time to time now. It's a strange one because he's the first young player in a while that I can remember that has been routinely told he needs to keep his feet on the ground publically by colleagues and coaching staff.
I also noticed that while Ten Hag celebrated his goal, when Garnacho took his shirt off, he went from celebration to hands on hips and shaking his head with a frown. So it would seem there's some form there for 'big time Charlie' behaviour and they're trying to knock it out of him to ensure he makes it before losing his way.
Mitten also said that he went to an open training session and Garnacho was tearing it up. When he spoke to the staff they said he was only doing that because he knew the media was there.
I think he'll start more regularly once they're all confident he's a team player and his dickhead behaviour stops.