The Whites suffered a first defeat in six Premier League games, going down 2-0 at Falmer Stadium.
"Fundamentally, it was due to a certain sector of the pitch where we couldn't prevent their three centre backs from playing the ball out cleanly, nor could we overcome them," said Bielsa.
"Our game in attack was very weak.
"I always think when we don't attack well, or defend well, it's a collective problem.
"The passing from back to front wasn't clean, nor could we win the ball back in their half, they were very organised.
"Their three centre backs protected the wide areas, they protected the backs of their two defensive midfielders, and they always managed to bring the ball back out when the goalkeeper didn't go long."