Palace had good chances at 0-0, but the game changed when Brentford was awarded a penalty.
Bryan Mbeumo's initial effort hit the post, but VAR ordered a re-take for encroachment, which he scored.
The Eagles were left chasing the game after Kevin Schade headed in a cross from Mikkel Damsgaard.
"I think the performance was ok, but ok isn't good enough," reflected Glasner.
"Especially two minutes before the penalty, we made too many mistakes in our own box. We couldn't clear the ball and then we cause a penalty.
"Everything went wrong with this situation (for the penalty), but the first three or four mistakes were our mistakes. No-one is to blame for it.
"Then when you are 1-0 down against Brentford it is difficult, we didn't create many chances today.
"The reaction after their second goal was great. We got a goal from Romain then we tried everything, but we were not quite accurate enough.
"We had some set-plays, but we were not good enough in these situations to score the equaliser - and that's why we lost the game."