My feeling was right about the tension between these two La Liga teams in the Champions League. I had no doubt picking the sending off market in that match because both teams have a history of intense encounters, especially in big competitions. The red card for Cubarsi proved it.

Cubarsi is a young central defender and even he already had some professional seasons with Barcelona, he is still very young and need to learn more, get more experience. In that situation, he began behind Giuliano Simeone and with that disadvantage, he could not defend well but he can try to run and follow that player and let Barcelona's goalkeeper does his saving job.
What he should do is just creating as most pressure on the Atletico Madrid player as possible but should never make such foul which surely results in a red card or a penalty. So he made a very bad decision at that moment, and it's more unlucky for him and Barcelona as immediately after that, Alvarez scored a free kick goal.