We need Walker and Cancelo in good form if we want to win a big trophy this season.
Rico and Ake can do a job at FB, but they are not top FBs (yet). We can get away with them at FB vs average teams (we didn't vs Everton though, both did nothing in attack). But we will struggle vs Bayern or PSG, and it will be difficult vs Arsenal too. Guess Pep will try Walker and Ake vs United, and that's probably the best he can do right now for Cancelo is poor, but the build up play may be sterile and painful to watch.
Something happened between Pep and Cancelo, and the team is suffering from it. Pep should play him at LB again. After the WC, Cancelo has two starts from 6 games, both on the right side, and one as a winger. He won't regain form while competing for minutes with Walker and Rico at RB. Ake is not the solution for the whole of the 2nd half of the season. Gomez isn't the solution either.