A frustrating afternoon at Stamford Bridge saw Chelsea's unbeaten run halted at four games as Brentford recorded a 2-0 victory.
The Blues dominated the opening period and created several opportunities. None were taken, however, and that proved costly in the second half.
Ethan Pinnock headed Brentford ahead shortly before the hour mark and, with Chelsea throwing men forward, the visitors grabbed a second on the counter attack through Bryan Mbeumo.
Pochettino made three changes to his side, albeit two were enforced. Axel Disasi came in at right-back for Malo Gusto, who dropped to the bench, while Enzo Fernandez and Mykhailo Mudryk were ruled out with minor injuries sustained in training.
That handed Noni Madueke and Nicolas Jackson starts and both were heavily involved during an opening 45 minutes that Chelsea dominated.
Chance after chance was created, with Cole Palmer dictating our attacking play. It was the 21-year-old who fashioned our first genuine sight of goal as he shifted the ball out wide to Madueke, who was left one-on-one with Aaron Hickey.
Madueke advanced into the penalty area and made enough space to guide a shot toward the far corner. Brentford goalkeeper Mark Flekken was beaten but the ball rebounded back off the crossbar.
Chelsea grew in confidence. Gallagher was next to threaten. The Blues skipper exchanged passes with Jackson and drove a shot toward the corner. Flekken, this time, was able to save.
He did so again to deny Cucurella. Sterling then thumped a shot high and wide from a tight angle. Both opportunities were fashioned by Palmer, who thrived in a free role behind Jackson.
Brentford were restricted to rare counter attacks forward; Robert Sanchez had very little to contend with in the opening period.
Yet for Chelsea's dominance – which included late efforts from Madueke and Palmer – the game was goalless at half time.
Pochettino's side needed to maintain their tempo in possession after the restart. Yet the Blues were sluggish and were made to pay just before the hour mark as Brentford broke the deadlock.
Mbeumo was the architect as he exchanged passes down the Chelsea left, flighted a deep cross to the back post, and Pinnock was on hand to plant a header beyond Sanchez.
A response was needed and Pochettino turned to his bench: Reece James and Ian Maatsen were introduced for Diasi and Madueke.
Brentford, with a lead to protect, dropped deep into a 5-4-1 shape and frustrated the Blues. Passes into the box were intercepted. Crosses were constantly cleared. Shooting opportunities were hard to come by.
Lesley Ugochukwu and Deivid Washington were brought on with ten minutes to play. Players were pushed forward, but Brentford defended resolutely.
Thiago Silva did steer a header wide from a Gallagher corner and, after a cutback from James, neither Palmer or Washington were able to make a clean connection with the ball inside the box.
Brentford continued to sit deep but carried a genuine threat on the counter and Sanchez had to produce a good stop to deny substitute Yehor Yarmolyuk. Mbeumo then flashed a shot just beyond the post.
Six minutes were added. In the final two Chelsea were awarded a late corner. Sanchez was sent up but the ball was cleared and Brentford countered through Neal Maupay and Mbeumo.
The latter was left with a simple finish into an unguarded net. The Blues were beaten.
What it means
The defeat sees Brentford move above us in the Premier League table. The Blues currently sit 11th.
The Blues return to action on Wednesday evening as they take on Blackburn Rovers in the Carabao Cup Round of 16.
Chelsea: Sanchez; Disasi (James, 67), Silva, Colwill, Cucurella (Washington, 81); Gallagher, Caicedo (Ugochukwu, 81); Madueke (Maatsen, 67), Palmer, Sterling; Jackson
Chelsea substitutes: Petrovic, Beach, Gusto, Badiashile, Matos
Brentford: Flekken; Roerslev, Ajer, Collins, Pinnock, Hickey (Ghoddos, 84) Norgaard, Jensen (Onyeka 62), Janelt (Yarmolyuk, 76); Wissa (Maupay, 76), Mbeumo
Brentford substitutes: Strakosha, Zanka, Mee, Olakigbe, Brierley