Why?
Why can't Pep experiment? He's probably trying to work it so Rico and Walker can play in the same side, keep Walker's recovery pace for the high line, and let Rico do the midfield work.
He's probably trying to extend Walker's career as well by making him diversify, and he's played plenty of games for England as a RCB in a back 3, trying out CB in a 4 is a logical next step.
We didn't lose the game because of Walker, he maybe could have done better on the second goal and he wasn't great in the build up, but what's the point of the sport if the most successful manager in the club's history can't move a player 1 position in the least important competition of the season?