While he had suspect moments, he also wasn't fully told to play the same way as Cancelo either.
Clearly Cancelo had more freedom to come inside and even push towards the box, while Walker was told to make a back 3 with Dias and Laporte, and let Zinchenko inside.
It's just that Cancelo is better than Zinchenko at the inside role, and even though Walker is a better defender than Zinchenko, Zinchenko is then more comfortable on the ball, it's a dilemma for Pep on how to rotate this perfectly (probably not possible).