Cucu would be a great oprion to rotate Cancelo and Walker next season. Given his and Walker's age, Cucu might become our main LB. But not straightaway.
As I see it, might be wrong of course, Cancelo and Walker will be first choice in the big games next season. Against weaker teams, Cancelo on the right and Cucu on the left might be the better choice, for we wouldn't need Walker's defensive qualities to the same extent.
Think Cancelo is our most valuable defensive player FOR Pep, because he is great in different positions and utterly fit. If Pep had to choose one defensive player to stay fit next season, it would be Cancelo, imo. A bad injury to him and we are fucked, given that his injury record is far superior to Walker's or Stones'.