Athletic made the most of their home advantage by making things very difficult for Barca.
Chances were few and far between, especially for the Catalans - who were limited to just seven shots and produced a paltry 0.4 xG all game.
Unfortunately for Athletic, though, one of those chances fell to the brilliant Lamal, who curled in a stunning effort from the edge of the area in the 68th minute to break the deadlock.
The home side produced more than double the xG but were unable to find a way past Joan Garcia and will feel like they deserved to take at least a point away from the game.
Barcelona needed the win to maintain their four-point lead at the top of LaLiga after Real Madrid beat Celta Vigo 2-1 on Friday night.
Los Blancos took the lead through Aurelien Tchouameni in the 11th minute before Borja Iglesias got Celta back on level terms 14 minutes later.
The match went back and forth and it appeared as if the points would be shared before Federico Valverde scored the winner deep into stoppage time at the end of the game to grab the three points for Real Madrid.
A late Ante Budimir strike rescued a point for Osasuna in a 2-2 draw against Mallorca at Estadio El Sadar.
Vedat Muriqi put Mallorca in a commanding position with a brace of goals in the 35th and 61st minutes and it looked to be enough before a late blitz by the home side.
Kike Barja pulled one back in the 89th minute and Budimir struck five minutes later to ensure the spoils would be shared.
Goals from Carlos Espi (50') and Joel Roca (90+4') cancelled each other out as Levante and Girona drew 1-1 at Ciudad de Valencia.
A brace of goals from Nicolas Gonzalez helped Atletico Madrid to a 3-2 win over Real Sociedad at Riyadh Air Metropolitano.
Alexander Sorloth opened the scoring in the fifth minute before Carlos Soler made it 1-1 four minutes later.
Gonzalez restored Atletico's advantage in the 67th minute but Mikel Oyarzabal equalised less than 60 seconds later.
However, the Argentine striker would add a second goal in the 81st minute to put Atletico ahead for the third time in the game, which they held onto for a 3-2 win.