Atletico will receive the results of a final round of testing on Thursday morning ahead of their quarterfinal clash with RB Leipzig at the Jose Alvalade stadium. Forward Angel Correa and defender Sime Vrsaljko will both miss the game after testing positive this weekend, forcing the team to depart for Lisbon a day late.
"On Saturday when we took the tests, we left training quite calm," Oblak said. "On Sunday morning, when they told us about the two positives, it was a bit stressful. Right now, it's common everywhere. Angel and Vrsaljko were careful and they have no idea how they caught it. I'm sure we're fine, today we had another test and tomorrow morning we'll get the results," Oblak added.
"On Sunday we were stressed and nervous for sure, waiting for the results to know how everything was," he added. "In these cases, a lot of people are asymptomatic and you don't even know you have the virus, but I'm sure we're fine and tomorrow morning there will be no surprises."
Atletico -- who reached the Champions League final in 2014 and 2016 but have never won the competition -- are being tipped as potential winners this year. They knocked out Liverpool in the last 16 with a dramatic 3-2 victory at Anfield and avoided the likes of Bayern Munich and Barcelona on their side of the draw.
"It's strange for us that people make us favourites," Oblak said. "We don't think about that. We know that Leipzig are a good team, who attack and defend well. It's a difficult game and I don't see either team as favourite in a one-off game. A lot of things can happen."
Leipzig coach Julian Nagelsmann has made headlines in Madrid after describing Atletico's style as winning "by any means necessary." The comments have been interpreted as critical, but opposite number Diego Simeone refused to be drawn on the matter in his pre-match news conference.
"I don't respond to the opinions of colleagues and I don't have any comment," he said.
"Tomorrow we'll play an important game against a team that play really well, with a young coach who's doing a fantastic job. They're a great team and the numbers show it. We respect them a lot. Tomorrow winning isn't just important, it's the only thing there is."