In the final minutes of the first half, Sergino Dest made it 0-2 to Barcelona. They had gone ahead through Antoine Griezmann and then Lionel Messi played in Dest and the American made no mistake. He hit the ball across Remiro and into the far corner.
Barcelona went to the Anoeta this Sunday to play Real Sociedad. It was a must win game for Koeman's side after Atletico Madrid had beaten Alaves to go seven points clear of Barca.
After a slow start from the visitors, Griezmann opened the scoring in the 37th minute after getting on the end of a rebound to make it 0-1. Griezmann hit home after Dembele's initial shot was saved by Remiro.
Just seven minutes later, Dest, who had had an effort turned onto the bar earlier, made it two before the interval.
Leo Messi passed to the American and he hit a shot across the goalie and into the far corner. Barca were 0-2 up at the break and Dest looked one of Barca's best players.
Dest, who has scored twice so far this season, got his first goal in La Liga for Barca.