Carlo Ancelotti's men sit third in the league table following this week's slip-up, trailing leaders Atletico Madrid by four points with just two games to go.
Victory for Diego Simeone's men and Barcelona this weekend will ensure the Spanish top flight becomes a two-horse title race and Ramos has been left to rue the "unique opportunity" which has passed his side by.
"It's a shame because with just two matches to go it's nearly impossible. It's practically lost now," Ramos told reporters.
"We've let a unique opportunity slip.
"We feel like we relaxed after we went 1-0 up and that was a big mistake. Focus has to be at a maximum and against a team whose safety is at stake you have to play it as if it were a final. They made the most of the only chance they had.
"The team's attitude was good, but perhaps we could have killed the game off."
Ramos, however, refused to completely give up on the domestic title so long as they can still mathematically leapfrog Atletico.
"As long as there is a mathematical chance, Real Madrid have to fight," he added.