The Spanish defender's header in the 31st minute was enough to earn a victory for the away side as it continue its efforts to escape the relegation zone.
In what was an open match from kickoff, both teams looked to take control of the game through the quick passing of their respective midfields and the play of their pacy forwards.
The home side was the first to create goalmouth action. Played through on the right, Oscar de Marcos reached the touchline and sent in a cross which found the feet of Markel Susaeta. The Spanish attacker had a glaring opportunity to open the scoring but had his shot blocked from inside the area.
Moments later, Fernando Llorente threatened Roberto Fernandez's goal once more. With his back to the net, the tall forward spun around his marker and sent a powerful strike just over the bar. Javi Martinez then forced a save from Roberto on the next attack as Athletic took control of the game.
Completely against the run of play, however, Granada opened the scoring. A swinging corner from Carlos Martins found Lopez, who rose above his marker to head the Portugal international's cross into the back of Gorka Iraizoz's goal.
The first 10 minutes of the second half did not follow the standards set by the action-packed first 45 minutes as both clubs had difficulties finding the final pass, keeping chances at a premium.
Although Los Leones continued to control the flow of play, the away side began to create opportunities to double its advantage. After a swift counterattack, Mikel Rico struck a shot from 25 yards which just scraped past Iraizoz's far post.
The Spanish midfielder was then sent through clear on goal but failed to beat Iraizoz as his curling effort with the outside of his foot went wide.
Athletic could not create any further opportunities after this and was dealt a surprise loss from the relegation threatened visiting.
With the win, Granada now sits on 12 points and has clawed itself away from the relegation zone. Bilbao, meanwhile, will be disappointed with the loss as the team missed its opportunity to temporarily climb to fifth place.