Karius is currently on-loan with Besiktas.
Asked if he received death threats in the wake of the Champions League final defeat, Karius told BILD: "There were some of them. But I can't take that seriously.
"These are people who write anonymously and don't even show their faces in their profiles. I don't think when people walk past me personally they can't open their mouths.
"Players receive some extremely hostile messages on the internet. If you read every message, you wouldn't be able to sleep for two days.
"It's insane what people come out with under the guise of anonymity, badly insulting others, discriminating against them and then becoming racist.
"You can't blame fans if they whistle a player. They pay admission and have the right to be dissatisfied. A professional has to endure that.
"But if there are personal insults or death threats, the limit is far exceeded."
The German admits he should have been more forceful in his public reaction to the criticism he received.
"Believe me, I've learned a lot from that!" he added. "In retrospect, I should have dealt with it publicly much more aggressively. I had a concussion after a blow from Sergio Ramos, which restricted my spatial vision.
"When the result was released, there was a lot of malice and insult, often well below the belt. I never used it as an excuse. But when people make fun of someone who has badly injured their head, I have no understanding.
"The reactions were over the top and disrespectful. My mistakes were not assessed fairly."