Former UFC champion Kamaru Usman was chillin' last night ... today he's prepping to fight Khamzat Chimaev across the world next week after Paulo Costa was pulled from the fight over an illness.
Costa and Chimaev were co-main event of UFC 294 -- going down October 21 on Fight Island in Abu Dhabi -- until Paulo came down with Staph infection.
The MMA star released a short statement explaining the situation ... "I am hospitalized and had to do a second surgery yesterday. According to doctors, a third surgery will be necessary on Friday.