Karius was at fault for two goals as Liverpool were beaten 3-1 by Real Madrid in the final, although doctors later confirmed the German suffered concussion during the match – potentially from a clash with Sergio Ramos.
Compatriot Trapp described continuing despite the concussion as "madness", and backed the Reds' shot-stopper to respond well.
"Then, that he played on, props to him for doing that. I had a concussion once and I couldn't play... at all.
"But maybe the adrenaline pushed him through and he didn't realise it. If it impacted him, I don't know, I wasn't in his body. But I know Loris a little bit. I know he will come back from that."
Even before the concussion was revealed, Trapp felt for Karius, acknowledging the errors could scarcely have come at a more costly moment.
"I know how it feels to make a mistake. As a goalkeeper, mistakes can happen, it is completely human," he said.
"It is hard for him, because he has had a great career thus far. He fought to be Liverpool's first-choice goalkeeper again, he got to the Champions League final, he performed well.
"And then this happens in the final, of all things. It can happen, but it is really hard in a game this big."