Usually when Cancelo plays at LB, Walker plays RB. They have complementary skills. Cancelo plays narrow, inverted. Walker plays wide, tries to get behind defenders and cut the ball back.
If Walker doesn’t play and Zinchenko doesn’t play either left back or left wing back then we will have very little width on either flank. It would be very difficult to overload one side of the pitch. It would hugely affect our style of play.
Unless Walker plays, we’re unlikely to have much attacking contribution from our right back. We shouldn’t add to that by taking away the attacking width that Zinchenko can bring at left back.