After that thrashing Pep could think two ways:
1. Rest the first team, they have a massive task on Tueday and they need to be fresh, we can win the league next weekend, feck united!
2. We’re out of the Champions league lets smash united and win the league Saturday and take our chances Tuesday!
I think he will opt for Option 1! Rest 5-6 players with Tuesdays task in mind.
Option 2. He knows they're probably out of the CL, he'll want the good news story of winning the league vs the local rivals to mitigate the disappointment/press reaction resulting from last night.
They'll play a strong side. We'll be looking to recreate Liverpool's second-half performance, sit deep, and keep them out/heap more misery on.