Even in his shite season and with less than double minutes Sancho has more goals, and scores/assists every 186 minutes against Grealish's 218 minutes.
Not to mention how easier it is to score or assist for City too.
Sancho is pretty average but this pretty much says a lot about Grealish, and he is playing in his favourite position for City and we would easily prefer Rashford there.
For stat nerds, their chances creation is similar too, and Grealish makes just few passes per game more. I mean, it's pretty obvious just by watching him, but this just confirms how shit Grealish is. He literally does nothing except dwells on the ball and gets fouled a lot.