Two goals within two minutes for Athletic in the first-half were enough to seal victory as Villarreal toiled after the sending off of midfielder Bruno Soriano on the stroke of half-time.
The scoring was opened on 33 minutes as Athletic right-back Iraola whipped in a superb cross from deep that was diverted past the keeper by Mikel Rico's head.
Minutes later, a horrible attempted clearance by Villarreal's Mateo Musacchio saw the ball fall to Aduriz, who pounced and finished clinically to double Athletic's advantage.
Despite dominating possession, Celta could not find the breakthrough and were left heartbroken by Diop's late strike.
Juanfran's free-kick from 25 yards struck the wall and found its way to Diop, who crunched a low volley into the corner.
