Brazilian forward Vinicius, 19, did not train with the squad on Thursday as a precaution ahead of Friday's La Liga home game against Alaves.
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"He has not tested positive, but it went wrong," Zidane told reporters in a virtual news conference.
"There was an error. There were errors on other occasions as well and that is why we followed the protocol and he did not train, for safety.
But it has not been positive. This afternoon we will do another test. If everything goes well he will return with the group to train and play.

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Real will be without Luka Jovic after Spanish media reported the Serbian striker was forced to self-isolate at home as a precaution after coming in contact with a friend who tested positive.
Zidane confirmed the 22-year-old, who has had an underwhelming debut season in Madrid and was recovering from a foot injury, was not available for the game.
"Since we do not want to risk it, he is not with us. I hope this changes quickly and that he comes back soon," Zidane added.
"These are things that happen. It's true that he's not very happy, especially with the injury. But he's a very good footballer, a striker who scores goals but has not played much.
"We will count on him and for next year too."
Real Madrid are top of La Liga with 77 points, one point ahead of Barcelona and with a game in hand on their old rivals.
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