Sevilla moved above Barcelona into second place in La Liga and level on points with Real Madrid at the top, following a 2-1 win against Real Betis.
With his side trailing at half-time coach Jorge Sampaoli further enhanced his reputation by making the changes which sealed victory for his side in the derby game.
Betis dominated the first half and took the lead from a free kick by Riza Durmisi in the 36th minute.
But Sampaoli's decisions during the break to replace Franco Vazquez with Iborra and an ineffective Pablo Sarabia with Wissam Ben Yedder completely swung the match in the visitors' favour.
Mercado equalised in the 57th minute, scoring from a rebound after goalkeeper Antonio Adan couldn't hold Adil Rami's point-blank header.
And Iborra completed the comeback from another set-piece when Steven Nzoni headed on a free kick for Iborra to toe past Adan.
Valencia failed to build on their impressive midweek win against Real Madrid by losing 2-1 to Alaves.
Carlos Soler put them into a 70th-minute lead but goals from Ibai Gomez and Aleksander Katai in the 78th and 86th minutes sealed victory for the hosts.
With two minutes remaining Fabian Orellana was dismissed for a second bookable offence.
Leganes overtook Deportivo La Coruna in the table with a 4-0 romp, pushing the Galician side to the edge of the relegation zone after their biggest win since joining the top flight for the first time last summer.
The small Madrid-based club got early goals from Alexander Szymanowski and Martin Mantovani. Unai Lopez and Alberto Bueno added late goals after Deportivo's Raul Albentosa earned a direct red card for elbowing Miguel Guerrero in the neck.
Leganes' first win in 12 rounds came a week after their gutsy performance at Barcelona ended in defeat.
Adrian Gonzalez scored on either side of half-time as Eibar maintained their excellent home record with a 3-0 win over Malaga, their eighth victory in 13 matches this season at Ipurua Stadium.
Sergi Enrich recorded his 10th league goal of the campaign to round off the win that pushed the modest Basque club into seventh place.
Roberto Rosales was sent off in the second half for Malaga, who are 14th.
Information from The Associated Press used in this report.