Well, if you really want brutal feedback from complete strangers, here goes

Good plot line, I wasn't expecting a mob story on an anime forum. The one main problem I had was the overuse of the words 'since' and 'which.' Quite a few of those sentences that were extended with these words would've sounded better if you'd cut them off and made new sentences. This sentence could be cut into three, and would flow with the story better(though you managed without those two words

) :
"The whispers from the crowd that Grant had overheard earlier told him that this man hadn’t been defeated for several weeks and from where Grant was standing, that didn’t seem too farfetched at all, but he knew that the man would go down tonight." Man that seems rushed and breathless! You could change it to:
"Rumors Grant had overheard earlier told him this man hadn't been defeated for several weeks. From where Grant was standing, that didn't seem too farfetched (or that didn't seem farfetched at all). It didn't matter, though, he knew this guy would be going down tonight." This sentence could be improved by taking out 'at him': "The crowd started to laugh at him because his body had become less toned and a little saggy from lack of nutrition." Tightens it up a bit, unless you have a certain word count you have to hit. This one: "Grant had been too focused on dodging and counter-attacking to notice that he had dodged too far back and his punch barely reached the man at all." might sound better as: "Grant had been too focused on dodging and counter-attacking to notice that he had dodged too far back, his punch barely reaching the man at all." Little things like that, to slow the reader down and make the story come through more clearly. Oh, and this one: "Grant’s anger was calmed instantly" is unusual (good, but unusual), but might make more sense if you said: "Grant was stunned." Excellent description of the fight, I could actually see it happening. The pace of the story is good, and like Timewiz said, the ending line was classic. Hope this was helpful!