Real Madrid is facing a difficult situation at the moment. They have to focus on 2 tournaments, which is draining their energy. They have to win when they meet Rayo in the next La Liga tournament, and in the next 2 days after, they have to win also when face Atletico. But Real Madrid has an experienced coach, and Anceloti often faces this complicated situation, and continues to get the trophy. Except if the squad isn't complete, yes, a key player like Bellingham should be playing in the next match to minimize the lag points. I'm not quite sure, has the suspension been completed? due to yellow card accumulation that make Bellingham can not playing on champions league, and also La Liga when meet osasuna.
Playing in both Champions League and La Liga will be harder for playing in only one competition, but remember that Real Madrid, Barcelona and Atletico Madrid all are playing in both competitions. Barcelona and Real Madrid had good advantage after their first leg matches and they have good opportunities to join quarter finals.
Assume Atletico Madrid will lose to Real Madrid and have to leave Champions League after round of 16, they will have sort of advantage than Barcelona and Real Madrid in La Liga competition. It will be their advantage but not a kind of guarantee for La Liga title. In many seasons, Real Madrid and Barcelona managed to play well and win both La Liga and Champions League titles. They have enough quality and experience to play well and repeat that succcess again in this season.