Atletico really underestimate Elche.
Moreover Atletico also didn't play as best as they are usually. Tjey cound't domimate the match and not really ready with the match. Seems like to be this. That is why theybwere lost.
Btw, because if Atletico loss and Real Madrid victory, now Real Madrid becomes on their second position as runner up in La Liga.
Atletico played not in their best form, they won the ball possessions but they don't get many chances to score. During the match, they only got 4 shots on target. Even, Elche could make more shoot attempts and they succeed to score 1 goal. With this result, Atletic must drop their position in 3rd place. Now, Real Madrid returned to 2nd place and probably stay there until the season ends. Atletico Madrid really wasted the chance to finish in 2nd place.
Atletico Madrid came with all their squad and they lost to Real Madrid that has to lay low some of their key players against the Champions League and they failed to exploit that opportunity from Real Madrid. However, despite the race, Real Madrid is not that far from Atletico, they are only two points ahead of Atletico, any single mistake will change the table in favor of Atletico, the only reason why I think Real Madrid wouldn't be concerned about the rest of the season is that they will finish among the top 4 because Real Sociedad has to win 3 matches to come close to Real Madrid and they have just 4 remaining matches, they can't go to Champions League with Real Madrid.