The German, who is known for his nightmare performance for Liverpool in the 2018 Champions League final, was parachuted into the starting line-up against Manchester United in place of the suspended Nick Pope and cup tied Martin Dubravka.
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The Wembley showpiece was his first match for an English side since his error-strewn performance in Liverpool's defeat to Real Madrid in Kyiv.
And if the 29-year-old wanted to banish memories of that display in Kiev five years ago, his warm-up would not have done him any favours.
As is normal before a match, a Newcastle coach took shots at the goalkeeper to get him ready for the game.
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But Karius did not impress, and was filmed spilling what appeared to be a routine effort in front of him, as well as letting a ball squirm under his body and into the net.
Before the match, Eddie Howe insisted he had not specially prepped the former Liverpool star for the final.
He said: "We have tried to be very normal with Loris. He's an experienced goalkeeper. He doesn't need too many words from me.
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"He's worked with the coaches and he's worked with the team. We've done all our prep as if he's our no.1. Now the rest is up to him to produce a good performance."
It was a different story during the match, however, as Karius acquitted himself well in the final, although he conceded two goals in the first half.
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Casemiro headed the Red Devils in front, getting on the end of a sublime free-kick by Luke Shaw.
Marcus Rashford thought he had doubled United's lead after Casemiro had headed home earlier in the match when he raced through on Karius' net, though his strike was later ruled as an own goal.
The Newcastle stopper was unfortunate to see the ball fly over his head after it took a wicked deflection off Sven Botman's boot.
Karius kept his side in the final as he pulled off an acrobatic stop to deny Wout Weghorst just before half time.
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Speaking on talkSPORT, Stuart Pearce said: "We've had a footballing treat here… a wonderful stop from Karius, he's kept his team in it."
Meanwhile, former Newcastle player Lee Clark said: "He couldn't do anything about the goals. He was so unlucky with the second goal, it took a deflection off Botman and the first one is a world-class delivery."
Karius impressed after the break, denying Marcus Rashford as well as brilliantly saving a late Bruno Fernandes effort.
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His teammates were unable to get back into the game as Newcastle went on to lose 2-0.
On Karius' performance, Magpies boss Eddie Howe said after the match: "Very pleased with Loris, he made a number of good saves. There were a number of moments where he stood tall… he can be very proud of himself."
Karius' efforts have been recognised elsewhere with stats company Sofascore giving him a 7.9 rating, the highest of any other Newcastle player.