Alaves spurned the chance to go top of La Liga on Friday with a 1-0 defeat at Leganes, who moved out of the relegation zone thanks to Youssef En-Nesyri's goal.
The striker fired home three minutes before half-time after Jonathan Silva cut the ball back into his path to delight a raucous Butarque.
Alaves, who started the season brilliantly but have now lost two of their last three games, remained fourth, one point behind leaders Barcelona.
"We have to give the boys credit, it's very hard for a team like Alaves to win in the top flight," Alaves coach Abelardo Fernandez said.
"It's a very tough competition, the best league in the world. Our team is having an extraordinary season. This was an equal game, we knew Leganes would be physical and direct."
En-Nesyri's strike earned Mauricio Pellegrino's Leganes a deserved victory, their first win in six games taking them 16th.
"We struggled at the start of the season but we are improving," Leganes captain Ruben Perez said. "Home matches are where we need to pick up the most points."
Champions Barcelona travel to third-placed Atletico Madrid on Saturday.