But before criticizing Jack, let's think a little bit -> what's his position...
So far Pep has played him either as LW or False9, but did he play that in Villa ? Nope
In an ideal situation, Grealish is a LCM or playmaker playing as a number 10 in 4-2-3-1 system, but as the later is too old now and only maybe Jose continues to play that, in Pep`s system he is LCM - no more, no less.
Grealish has amazing "ball retention" skills and can be our second Bernardo.. Actually I think that Bernardo-Grealish duo can be City`s xaviesta but let's not involve my dreams and hope here :D
So, in this formation - I think he will be very good
---------------------Rodri--------------------------
-----Bernardo------------------Grealish----------
So it's not about Jack and his quality, he isn't playing the position - where he does his best.
He is not a winger or a striker FFS, he is a midfielder and does that job pretty well :)