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.
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"They were superior and played better, there is no excuse, football is simple," the midfielder said, via the club's website.
"In La Liga every game is difficult.