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It's true that Real Madrid is already 8 points ahead at the moment, but they can't be too dismissive and think they will easily win the La Liga title because there are still 10 matches left in this season which makes anything possible and even Real Madrid still has several difficult matches left. including against their rivals Barcelona.
Real Madrid will definitely be distracted by the UCL match against Manchester City, but I am quite sure that a team as big as Real Madrid will be able to prioritize both equally without leaving anyone out because both are very important to them.
If you think Real Madrid will underestimate any opponent in the remainder of the season, then I guarantee you are wrong. Then from that, this season only leaves 9 matches instead of 10 matches with an 8 point advantage over Barcelona. Of course it's still not safe, but to make some rotations to focus more on the Champions League, I don't think it's a big problem.
Ancelotti knows exactly about your concerns about the potential for Real Madrid to fail to win the title without the threat of Barcelona. Bbut I believe that winning 8 points is a blessing in itself for Real Madrid to remain comfortable in every remaining match this season, even for the El Clasico match against Barcelona.