Two disallowed goals proved costly as Jonathan Woodgate was denied on his Riverside debut as head coach.
Woodgate’s side dominated the first half against Brentford, but with two efforts ruled out during the opening 45 minutes, they were vulnerable to a visiting sucker punch in the second half, and ended up falling to an undeserved 1-0 reverse.
The first moment of controversy came when Boro had an early goal disallowed. With the Brentford goalkeeper stranded near the corner flag after attempting to keep the ball in play, Lewis Wing whipped over a quickly taken corner which was deflected into the net by a visiting defender. The goal was immediately ruled out, though, after an adjudged foul by Ashley Fletcher.
The injustice raised the decibel levels Inside the Riverside and Paddy McNair sent a powerful strike skidding just wide of the upright while David Raya had to be alert to intercept a Jonny Howson ball perfectly struck for Britt Assombalonga.
Boro had the ball in the net for a second time before half-time when Daniel Ayala picked out the run of Fletcher with a great ball, but his deft finish was this time ruled out by an offside flag.
As at Luton last weekend, Marvin Johnson was unlucky not to get off the mark when he was denied by a finger-tip save from Raya after showing great tenacity to win the ball back for Boro before working himself the opening.
Assombalonga was next to curse his luck after he did well down the left, beat Raya, but saw the ball come back off the inside of the post from the unlikeliest of angles.
Darren Randolph was called upon to make his first real save of the game early in the second half, and in typical fashion, the Irishman pulled off a great stop to deny Mathias Jensen. Henrik Dalsgaard also had a shot deflected over the bar as Brentford showed their qualities with the ball.
It was during this spell that Brentford snatched the lead. The lively Sergi Canos cut in from the right and his low ball into the middle was turned home by Ollie Watkins.
Boro responded well, Wing having a free-kick blocked and McNair seeing a powerful strike deflected away from goal. Assombalonga was also denied by some good defending from Julien Jeanvier on more than one occasion.