It's clear since Pellegrini last season that those positions (both wings) were upgradeable. Perhaps Pep thinks his philosophy can do miracle & prolong these outgoing fullbacks playing them as some weird narrow midfielder or as central defenders at one point or another during the past season?
Signing one or 2 back last summer would mean they can both get better value & in better negotiating position come this summer. Remember Dani Alves did move last summer with Pep was announced way before the transfer market window open.