Missing Walker isn't just missing a player, it's missing our primary means of defending counter-attacks. Locking down his flank and also having the pace to cover across when needed is a huge part of how we defend.
Sure, he makes mistakes, but no one else in our team can physically do what he does. If Walker's out for any serious amount of time then I'm guessing Aké will play as a very conservative left-back and drop almost into a back three like Abidal did for Pep, with Cancelo playing the Dani Alves role and staying more advanced