Liverpool eventually won 2–1.
Home side Brentford took the lead in the 43rd minute, but just before half-time, Brentford midfielder Jehor Jarmolyuk was sent off.
With an extra man on the pitch, Liverpool took control. Victor Muñoz equalised in the 63rd minute. Barely ten minutes later, Isak coolly placed the ball inside the far post after being played in by Frenchman Bradley Barcola.
The home side were further reduced in stoppage time, when midfielder Vitaly Janelt also received a red card.
It was Isak's first match since he was injured during Sweden's match against Romania in the Nations League on September 25.