Chelsea game PSG has will decide a lot about the league as well. Not only this one, which would be a normal game, and they could be tired and that could lead to them not be able to play their best players and need to rest some of them. Because if the game is close, like a tie, or even if Chelsea wins it, which would be shocking, then PSG will rest their players on the weekend and create issues for the league game.
On top of that we could see the second game possibly go as much as overtime, and on that 120 minutes they would be tired once more, putting yet another league game at risk. None of this could happen, they could easily get a win in the next 2 league games with 2 UCL games and just be fine, but the risk is there.
I completely agree; the risk of fatigue always exists when a team is competing in several competitions simultaneously. Important matches like PSG vs. Chelsea can be very physically demanding, especially if they are close or go to extra time. In these situations, it's common for the coach to rest some players during the league, which can ultimately influence the results.