Isak returned after missing last week's defeat to Manchester City as Liverpool shaded the early openings, though the initial problem for Liverpool was more that their expensive attack was yet to gel.
Brentford responded with a series of fast breaks and Liverpool had to be strong, soon making Liverpool rue a failure to clear their lines after being pumped up by a VAR intervention.
Yehor Yarmolyuk was never going to get away with it in the video assistant referee era, and Liverpool made the midfielder pay.
If Liverpool's spluttering forwards end up remembering this as the day when their luck turned, then they will owe a big debt of gratitude to Yehor Yarmolyuk.