Neal Maupay put Brentford ahead on the stroke of half-time when he curled a stunning 25-yard drive past Leeds keeper Kiko Casilla and Sergi Canos put the game beyond doubt just after the hour mark when he finished off a slick four-man move.
The result leaves Leeds three points behind second-placed Sheffield United and with an inferior goal difference, with just two matches remaining.
Bielsa said: “I am very sad with today’s loss and it’s going to be very difficult but as long as maths say something we will follow this. But we know our opportunity is now less.”
The Argentinian shook every one of his players’ hands at the end of the pivotal game “because we all saw that our possibility to finish first and second were decreasing… and the players felt that too. I thought they deserved that for all the effort they made.”