After the subs were made, our midfield 3 was 97 years old (Gundo, Kev and Bernardo). Added to that, Walker has essentially had 1 decentish cameo this season, but is mostly just a liability. Aside from longer term squad needs, Pep needs to move on from his loyalty to players and just think about what is best for the team.
At 4-0, 20-25 minutes for Gundo in a midfield without covering legs, does nothing for him, doesn't add control and is at the expense of development minutes for squad players. 20-25 minutes for Walker (with everyone in defence moving round) does nothing for him, makes the team weaker and is at the expense of minutes for other players. They have both shown what they can and can't do this season.