Good luck to him.
Sad to see him go because he's one of us but, in all honesty, personally, I never really saw the big deal. He's clever enough in possession and looks dangerous enough when he cuts in on his left into some space. He understood Pep's methods well enough. But he was also really limited compared to where Foden was at the same age and he's never really showed that he might become anything more than a decent prospect. Still gets shrugged off the ball too easily, doesn't have the nous yet to make up for his lack of pace, pretty indecisive when he's in sight of the penalty area.
Obviously all players progress differently but 13 senior career starts at the age of 21 isn't anywhere near enough - he's running out of time to deliver on his potential. By this stage Foden was playing a starring role in a Premier League-winning side that was reaching the latter stages of the Champions League. Palmer's still at the stage where he needs one of Foden, Grealish, Bernardo, etc. to get injured so he can have a proper run. Now Doku's come in I don't really see where he gets a chance.
Don't really see how going to Chelsea gives him more of a chance of regular football either - think he'd be more suited to places like Brighton, Fulham, West Ham, etc. Big clubs with smaller squads who might need him a bit more. Chelsea have got so many wide forwards already - Sterling, Mudryk, Madueke, Jackson, Washington, Nkunku - so where does he fit? Hope it works out for him but I honestly can't believe we've just got £45m for him when, at best, I thought he was gonna go for about half that or head out on loan to somewhere mid-table.