The Reds were misfiring as they trailed to Keane Lewis-Potter’s first-half goal, but the match turned on a needless red card for Brentford’s Yehor Yarmoliuk on the stroke of half-time.
Victor Munoz hauled Liverpool level after the break before Isak struck with his fifth goal of the season. Brentford then lost Vitaly Janelt to a second yellow card in stoppage time.
Swedish striker Isak had missed the previous two matches but was able start alongside Florian Wirtz and recent signing Bradley Barcola in a £350million frontline.
The £123m wideman attempted to chip onrushing Bees keeper Caoimhin Kelleher but made an almighty mess of his effort, tamely stabbing the ball wide to prompt a chorus of ‘what a waste of money’ from the home fans behind the goal.
Back with a bang: Alexander Isak’s winning goal for Liverpool spared Bradley Barcola’s blushes
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But very little else had happened in a pretty low-key affair in west London, until the match exploded into life shortly before half-time.
Liverpool were awarded a penalty after Dango Ouattara looked for all the world like he had tripped Alexis Mac Allister in the area.
However, a VAR review showed the Bees attacker had got the slightest touch on the ball, and after a check on the monitor referee John Brooks overturned his decision.
Two minutes later Brentford took the lead when Lewis-Potter, among the smallest players on the pitch, met Igor Thiago’s cross with a bullet header into the top corner.
But in stoppage time Brentford fell foul of a VAR decision, when Yarmoliuk was spotted to have aimed a ‘headbutting action’ towards Jeremy Jacquet as the pair grappled at a corner.
The Ukrainian’s red mist earned him a red card, although Liverpool were not awarded a penalty as the ball was not in play.
Nevertheless, Liverpool made the most of their numerical advantage in the second half.
They equalised when Wirtz sent Munoz racing down the right and the Spanish winger turned inside Lewis-Potter before arrowing a left-foot shot past Kelleher.
The winner arrived in the 70th minute when Mac Allister swung a deep cross towards the far post, Barcola headed the ball back and Isak took a touch before lashing it past Kelleher.
To rub salt in the Brentford wounds, German midfielder Janelt was sent off in stoppage time for pulling back Rio Ngumoha.