Harvey Barnes gave Newcastle an 86th minute lead in the first-leg of Tuesday's round of 16 clash before Lamine Yamal converted a 96th minute penalty won by Dani Olmo.
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Cubarsi said at the final whistle: "We maybe weren't that good up front, but we had phases of the game where we played well and had possession.
"Sometimes you have to play these games by sitting back or playing as a block in midfield to get the result."
The defender added, "A small detail cost us a goal, but we have very dangerous players up front who can cause problems at any moment, and that's what happened. Dani (Olmo) came on, did what he had to do, won a penalty and we got a draw, which is a good result.
"At home, we have to go all out."