"The only way to improve is to get the players to work, to find confidence, above all in these complicated situations in games. It was a tough game, La Real started well," Atletico manager Diego Simeone said. "Right now, the dynamic isn't good, it needs to change. We need to think positive, I'm optimistic, I believe in my players, but obviously this isn't comfortable.
"It's a football problem, every day isn't the same, every year isn't the same, every month isn't the same. Last season was magnificent, just five or six months ago, and this season has been very difficult so far. The possibilities and opportunities are there, we have the league and the Champions League."
"There were situations where we could have scored before we conceded," Atletico defender Mario Hermoso said in a post-match interview.
"I think that there could have been a foul [for Januzaj's goal] from the cross. But we know how this is. We've had various situations where the same thing has happened, we should have got decisions that went against us, I don't understand it. Referees have to control things. There were ten substitutions and added on just three minutes. I don't understand."
"The team showed that that was an important game. We had our chances in the game and we didn't take them. That affects you. When you make mistakes and you concede, everything is more difficult."
"Our fans know that this is Atletico Madrid. We'll give everything in every game that we play. Right now we aren't getting the results, but we'll come back, we're the champions of Spain and we'll keep fighting."
The Basque side joined Valencia, Real Betis, Rayo Vallecano, Mallorca and Cadiz in the quarterfinals while Real Madrid face Elche and Athletic Bilbao host Barcelona in the last two round-of-16 games on Thursday.
Information from ESPN's Alex Kirkland contributed to this report.