The Spain striker intercepted Julian de Guzman's long pass back to his keeper to net his 19th goal of the campaign before picking up a second yellow card 14 minutes from time.
Depor made immediate use of the extra man when Piscu headed in to deny Valencia a return to the Champions League places, leaving them in fifth with 39 points.
Leaders Barcelona, who were beaten 2-1 at home by Espanyol on Saturday have 60 points with 14 games left. Their lead over second-placed Real Madrid has been cut down to seven points. The champions hammered Real Betis 6-1 at home.
Sevilla are third with 44 points after they beat Atletico Madrid 1-0 at home, and they are three ahead of Villarreal, who beat Sporting Gijon 2-1 also on Saturday.
Earlier on Sunday, Malaga moved up to sixth with 39 points, behind Valencia on goal difference, with a 3-1 victory at Real Valladolid.
Albert Luque put the promoted side ahead early on and Apono buried a penalty in the 29th minute after Eliseu flung himself to the ground.
Valladolid put them under pressure after the break hitting the woodwork twice and a Pedro Oldoni header briefly threatened a fightback, before Nacho made it 3-1 near the end.
"We have made it to the UEFA Cup places playing well, with the backing of the fans, and enjoying ourselves. It's the fruit of our own hard work," Malaga coach Antonio Tapia said.
Atletico are seventh with 36 points, ahead of Depor on goal difference.
