Sebastien Haller and Andriy Yarmolenko struck the goals for West Ham's 2-0 win.
Farke said, "At this level small details cost. This is a game where VAR didn't work. There was a nasty tackle against my centre-back [Christoph Zimmermann] and it wasn't even given as a free-kick.
"Then a few minutes later my centre-back wasn't able to spring back and block the cross [for West Ham's first goal]. My centre-back is now in hospital. My feeling was this made the difference.
"I'm not claiming for red cards, it is up to the VAR to judge it. The only thing I say is that my centre-back is in hospital and the other player is on the shoulders of the West Ham goalscorer. I'm a friend of VAR but on this day it didn't work."