He not only wants fullbacks that can defend, but also be holding midfielders. This is also the problem with Walker. He's fine defensively, but can't do what Zichenko does in an inverted fullback role with the ball. Waker's not good at playing out from the back and when in the opponents half he's not good at moving the ball and distributing it into the front players. Then you have Zinchenko who can't defend and get back into position when we're being countered on. Players like Semedo and Alaba would be all around better in every aspect for an inverted fullback.
I think how we set up next season will hinge on what happens with Cancelo/Semedo. Probably more 3 at the back if Cancelo stays and more a back 4 if Semedo were to come.
Semedo is exactly the same type of full-back as Walker. Zinchenko not being fast is fine, that's not the problem with him. He Can't stay out wide like a full back if our play demands it(like when we can't go thru the middle) and of course he's also not a natural defender.
Cancelo Is the prototype of full-backs that Pep wants, the Dani Alves type. Cancelo just has to learn to play that Inverted Role, and once he does, he has the exact skill set required to succeed as a Pep Full-Back.
But of course it's not easy, especially when the team is so unbalanced and fragile.