Thomas Tuchel said that the Swedish referee at the heart of Arsenal and Bayern Munich's 2-2 draw admitted he should have awarded the visitors a second penalty.
Harry Kane converted the first after 32 minutes to put Bayern 2-1 in front, but in the 67th minute, another incident happened which has sparked controversy.
Referee Glenn Nyberg blew his whistle, and Arsenal goalkeeper David Raya made his goal kick to Gabriel, who handled the ball, before kicking it back to his No. 1.
Tuchel told TNT Sports: 'I think the referee did not have the courage today to give a deserved penalty in a bit of a crazy and awkward situation.
'But he admitted on the pitch that he saw the situation, and that a quarter-final is not enough for him to give a penalty for a kid's mistake.
'So he admits that he knows about the mistake the player made. That's a bit frustrating.'