Foden and Mahrez are the only really known quality wingers in this system.
Alvarez has not played a minute of European football, and we don't know how he will adapt and we could just as easily be looking at loaning him out come January if he doesn't work out immediately.
Cole Palmer has not even played 90 minutes against a premier league side in a meaningful game. If Mahrez blew his ACL in our first pre-season game is anyone confident enough to say Cole Palmer could start week in and week out and put up Mahrez-like numbers and have the impact Mahrez does?
Grealish didn't exactly have a great season, I think he's got another year to improve and make progress. But it's still unknown whether or not he can adapt.
We need to find a pacy winger who can score IMO before the window is over.