Right now, Real Madrid is down 3-1 to Girona in a midweek league match. You might think they are heavily rotated as they have given up on the league title, but they aren't. Courtois is the only notable absentee. I think there is also an element of not playing full intensity.
If you watch City at all, you can see the exhaustion in KDB - his body just can't recover to play full intensity. I suspect this is what has happened to Liverpool as well - Klopp's style is just so demanding physically.
Interesting example you gave as I'm not sure those UCL fixtures are that much tougher than the Europa ones and you only have to get second.