I think it's true that Manchester City's main target in the Champions League is their first title, but that doesn't have to cost them the Premier League title which I think is just as important to have every season.
Both are important but at some point, manager gotta make sure that their players are fit for the next game so instead of pushing his main lineup then he might make some adjustment to it. These small adjustment and substitution might affect the team so they could probably lose a game or get a draw at some point on the league.
Pretty sure Pep is going to keep his main lineup fit for UCL game though