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I think vinicius needs to stay put and not play in this match as another important match awaits them, just like you said. Celta Vigo isn't really a tough one for Ancelotti's current squad, especially since carvajal isn't in play for the time being as well.
It is too risky for Ancelotti to play Vinicius or another player who is just recovering from injury in a match that may not be too tough. Celta Vigo is certainly not a team to be underestimated, but playing Vinicius and several players who are just recovering from injury would be very risky. The match against Dortmund certainly takes priority, but if Ancelotti finds his team having difficulty winning that match then he will likely play that too.
Thibaut Courtois and Andriy Lunin are expected to recover from 16 to 18 October, perhaps the same as Éder Militão. Real Madrid is finally ready to face any opponent even though they are being hit by a storm of injuries. There is certainly hope of winning the match against Celta Vigo, Madrid even has a high probability of winning.
The win probability is huge and that shouldn't be ignored... I'm not a Madrid fan whatsoever ( even if we all wanted to wager on teams base off of the fact that we're in love with their setting) I wouldn't even put my money on celta to defeat Madrid.
I can't match the present Madrid squad with the squad they had 10 years ago,.but I can say it's pretty much easier for them to defeat Barcelona than the other way round.
Real Madrid are favorites in the match against Celta Vigo, but when they play Barcelona for El Clasico, victory will definitely not be easy. I don't have any sentiments towards Real Madrid, but saying they can win easily against Barcelona would be a mistake. In fact, Real Madrid and Barcelona beat each other in many seasons and whoever wins the El Clasico has a good chance of winning the title too.