Erling Haaland needs to rest because he has contributed fully in every game and often brings victory to Manchester City. Liverpool shouldn't be difficult because Manchester City's strength already looks like it will be easy to beat them. At the very least, there are still a few days to prepare for Manchester City, and perhaps Erling Haaland will play some minutes.
If Erling Haaland is going to rest, then it shouldn't be against Liverpool, to be honest I think Liverpool is a dangerous club, now that everyone think they are not in form but they are still surprising people, one of the greatest mistake Manchester city will make is underestimating Liverpool, I don't really know why you think Liverpool won't be difficult for Manchester city to win, i dont really believe it will be a easy win for Manchester city, it will be a very tough match between Manchester city and Liverpool.
If you think about it, it's true that Manchester City will experience fatigue and its players, because in the previous few matches they played optimally and brought victory, including Erling Haaland who often becomes a sharp spear for Manchester City, of course if he's a human he might experience fatigue. Due to the tight match schedule.
Yes, Liverpool should see from this side and maximize their strength when they meet in the match with Manchaater City, but on the other hand, Manchester City also has the urge not to be far away and try to catch up with Arsenal who are at the top of the standings.