Atletico Madrid captain Gabi said the team were going through their worst spell under Diego Simeone after Monday's 3-0 defeat at Villarreal.
Atleti have now lost four of their last six La Liga games to fall 12 points behind leaders Real Madrid.
The defeat at El Madrigal equalled the team's worst result since Simeone took charge in December 2011, and they now have their lowest points total after 15 games under the Argentine -- 10 fewer than at this stage last season.
Speaking on Cadena Ser's El Larguero radio show after the game, Gabi said: "It is the most difficult [run], for sure.
"We have had five years of total success, but we need to show that we are a real team.
"Mentally we have to clear our heads, to return to be being this ambitious team which is motivated to win, as soon as possible.
"The only way out of difficult situations is to work hard and be together."
Atletico are now sixth in the table and Gabi said challenging for the title "is going to be very difficult now for sure."
He added: "We should not think about such long-term objectives now. Our objective is to win our next game, to beat Las Palmas [on Saturday].
"Keeping clean sheets used to be our strength. We have to work together more than ever to defend better and take our chances in attack."
Atletico missed chances against Villarreal, with Koke hitting a post at 0-0 and Antoine Griezmann failing with an opportunity at 1-0.
"We're annoyed after the result, which was not what we deserved, but it was a heavy defeat and we're leaving Villarreal hurting," Gabi added.
"We are not having luck in front of goal. We hope this changes as soon as possible.
"All we can do is keep working to turn this situation around. It's clear that criticism will come now, but we must all be together.
"Now we must be more united than ever, not worry about the criticism or look outside."