Karius has not played in a competitive match for Liverpool since the 2018 Champions League final against Madrid when he made two errors that helped cost his side the trophy. The 26-year-old is now on loan at Besiktas until the end of the season and remains under contract at Anfield until 2022.
The German goalkeeper said that he received death threats after he gifted crucial goals to Madrid's Karim Benzema and Gareth Bale. He was later revealed to have played on with a concussion following a clash with Madrid defender Sergio Ramos.
"I should have been more aggressive about this in public," Karius told Sport Bild. "I was just delighted that I knew what was going on during that match [after the concussion]. I did not want to make it public. And when the results were published, I received a lot of malice and insults, below the belt.
"I never used it as an excuse. But I lack all understanding for people mocking someone who suffered a severe head injury."
Karius said that his role in Liverpool's path to the Champions League final was forgotten by fans after the game and was instead subjected to "respectless and excessive reactions."
"Yes. They [threats] existed. But I can't take them seriously," he added. "Those are people who write anonymously and not even their face in the avatar. I believe that if those people see me in public, they will not say a word."
Karius was replaced as Liverpool goalkeeper by Alisson the following summer, and the Brazil No. 1 was part of the club's 2019 Champions League-winning side in his debut season.