Besiktas manager Senol Gunes claims that "something is wrong" with Loris Karius following the goalkeeper's performance in a 3-2 win against Konyaspor.
Karius was deemed to be at fault for Omer Ali Sahiner's 20th-minute equaliser, conceding his 37th goal of his first campaign with the Turkish side.
The German goalkeeper failed to make what should have been a routine save and was then jeered by his own fans for the remainder of the match.
The goalkeeper made two crucial errors in that match, as Liverpool fell 3-1 to Real Madrid, and his coach says his time at Besiktas has done little to repair his confidence.
"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."
Rumours have suggested that Karius wanted to be removed at half-time due to the jeering, and Gunes did little to ease those rumours.
"It has been like that since the beginning," said Gunes. "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."
In the wake of Karius's struggles, the club has denied reports that he could be dropped for Tolga Zengin.
The 35-year-old back-up is not currently a first-team player, but Gunes says he is certainly considering bringing him back in.
"Something is wrong, he has also been unlucky," Gunes said of Karius.
"[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."
Besiktas, who currently sit third in the league, take on Goztepe on Saturday.