Gunes admits that he would drop the under-fire German if 35-year-old deputy Tolga Zengin was available for selection.
He conceded another soft goal, failing to make a routine save for Omer Ali Sahiner's equaliser on Sunday for Konyaspor, and was jeered by his own fans.
Gunes offered no reassurances to the 25-year-old when he appraised his display.
"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 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.
"Something is wrong, he has also been unlucky. [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."