During the Bellator 181 main card, it was announced by the promotion that Mousasi would make his debut against Alexander Shlemenko at Bellator 185 on October 20, 2017. In the opening minute a punch from Shlemenko broke Mousasi's right orbital, causing Mousasi to fight with one eye for almost the entire bout. The rest of the first round mainly consisted of Mousasi scoring multiple takedowns, landing many strikes, and attempting several neck cranks and rear naked chokes. A Doctor inspected Mousasi's eye before the second round and deemed him fit to continue. In the second round, both fighters landed strikes in short exchanges for the first three minutes until Mousasi landed two takedowns and spent most of the last two minutes making submission attempts and landing strikes, which caused Shlemenko's face to bleed. When the round ended, Mousasi was still in a back mount dominant position over Shlemenko. In the third and final round, Schlemenko landed the better of strikes standing up and Mousasi's only takedown attempt was unsuccessful. Mousasi won the fight via unanimous decision with the scorecard of 29-28, 29-28 and 29-28. Seven out of eleven MMA media outlets scored the fight as a decision win for Shlemenko.