Karius has struggled for form since joining Besiktas on a two-year loan deal from the Reds back in August – keeping just five clean sheets in 26 games.
The German’s struggles continued again on Sunday as he conceded two easy goals in a hard-fought 3-2 win over Konyaspor and was jeered by his own supporters.
"He was at fault for the goals he conceded," Gunes said.
"Karius has gone a bit stagnant, something is wrong with his electricity, motivation, enthusiasm for the game.
"It has been like that since the beginning, "he added. "He does not really feel a part of the team.
"It's something we haven't been able to work out and, of course, I am partly to blame for this."
Gunes further suggested he would drop the 25-year-old for his poor form if he had suspended 35-year-old Tolga Zengin available for selection.
"Something is wrong, he has also been unlucky," Gunes said.
"[He] is talented but it hasn't worked out and we have a problem. Let me put it this way, if I still had Tolga available I would play him."
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