On November 4 , it was confirmed that Lesnar was suffering from mononucleosis and that his bout with Carwin would have to wait a bit longer and the fight for Lesnar 's heavyweight championship was cancelled . On November 14 , at the UFC 105 post @-@ fight conference , Dana White stated , " [ Lesnar ] ' s not well and he 's not going to be getting well anytime soon " and that an interim title match might need to be set up . In addition to mononucleosis , it was revealed that he was suffering from a serious case of diverticulitis , an intestinal disorder , which required surgery . After further diagnosis , Lesnar underwent surgery on November 16 to close a perforation in his intestine that had been leaking fecal matter into his abdomen , causing pain , abscesses , and overtaxing his immune system to the point that he contracted mononucleosis . From the level of damage to Lesnar 's system , the surgeon estimated that the intestinal condition had been ongoing for around a year .