Madrid were stunned in a 2-1 defeat on Sunday, leaving them second to rivals Barcelona in the table with 11 matches remaining.
Casemiro said it could be the result that denies Madrid, who are two points adrift of Barca, the league title.
"They were superior and played better, there is no excuse, football is simple," the midfielder said, via the club's website.
"In LaLiga every game is difficult. To win LaLiga we have to play well in every game and we didn't play well.
"We have to keep working hard, we have not been up to the task and we have not played like we did against Barcelona.
"We are human too, but it could cost us the league title. We have to look at Eibar now."
While Karim Benzema's penalty cancelled out Sidnei's opener in the first half, Cristian Tello scored an 82nd-minute winner for Betis.
Madrid defender Raphael Varane urged his team to continue fighting late in the campaign.
"We lacked a lot of intensity, especially in the first half. We didn't play at the pace we wanted to and we let the opponents have the ball," he said.
"We're dropping important points, but we're confident we can fight until the end.
"We're trying very hard to get back to the top of the league and today was a drop in quality from everyone.
"We still have a lot of games left to play and we're confident we're going to get that intensity back."