Going full strength doesn't guarantee a win.
Chances to win are like: full strength 80%, 2-3 changes 75%, 4-5 changes 70%.
It's not worth to risk injuries and fatigue before Real in order to make the win at Everton 10% more likely.
Plus, it's not a given that the likes of Foden, Mahrez and Alvarez would perform worse than the respective starters in Madrid. They'll be fresh, raring to go and won't try subconsciously to avoid injuries. If things don't go to plan, Pep can sub them off and play, say, Rodri, KDB and Grealish in the final 20-30 minutes.