Zinchenko is a left full-back, Fernandinho plays at the backline he was converted from DM to CB last season where he performed so well, Sterling, Mahrez and Silva are wingers and Grealish would have no business on the wing, Rodri is no better than Grealish so this leaves the midfield wide open for Grealish, Kelvin and Gundogan to operate so I don't see any challenge for Grealish if the deal went through.