Onana was chosen to start in goal against Fenerbahce by head coach Fatih Tekke during their Turkish Super Lig match on Sunday.
Unfortunately, Trabzonspor experienced their initial defeat of the season, with their new loan player responsible for the only goal in a 1-0 loss.
The goalkeeper managed to touch a low shot from just inside the area but failed to maintain possession of the ball, which did not seem to be heading on target.
Fenerbahce player Sebastian Szymanski reacted quickly to the loose ball and passed it to Youssef En-Nesyri, who scored easily from close range.
Trabzonspor, who were reduced to ten players following a red card for Okay Yokuslu, concluded the match with a two-game winless streak.
Despite the blunder and the loss, Onana put in a solid performance throughout the match.
The 29-year-old recorded eight saves for Trabzonspor against 29 shots directed at him from Fenerbahce.
Even with his mistake, he earned the title of player of the match. Trabzonspor will be hopeful that Onana can build on his performance from the weekend when he makes his home debut next weekend against Gaziantep.