Rodri has started 91 games in the last 2 seasons, he's not being rotated. He's very occasionally being rested in games Pep thinks we can get away with it, or aren't important, the same way Lewandowski was rested for 10% of games in the example I gave.
Semantics. Players like Kevin, Rodri, Bernie, Walker get rotated in some games to avoid injuries and maintain their form, not because they are not key. Precisely because Haaland is key (but also because we have Alvarez as well), Haaland won't be played more often than necessary, and will be rested probably more often than expected. He won't start every (league) game he's fit for, especially after big CL games March-May. My guess is he will start 30 league games injuries permitting.