I think it's both. In a sense, NDombele's style of play means that he would require plenty of energy, which is taxing over 90 minutes especially in PL, where teams rarely give in thus allowing you a breather.
NDombele may be able to last 90 minutes, but he may not be as effective throughout the game. And we have a great option in GLC to rotate. Getting them both running 100% when they're on the pitch, while conserving them preventing potential burnt out is a good idea. The season is long and congested schedule, so there is no point in taking any risk now, especially when we manage the game well. Fan service can wait.