Brock Lesnar's longtime friend and Jiu-Jitsu coach tells TMZ the hospitalized fighter WILL return to the Octagon -- and he guarantees Brock will return to battle condition in the next six months.

We spoke with Rodrigo "Comprido" Medeiros -- a 7-time World Jiu-Jitsu champ -- who tells us the reigning UFC heavyweight champ "is getting better and that is the most important thing ... He will be fine and be fighting again within 6 months."

Medeiros adds, "I can just say that the heavyweights can stop laughing and get ready to start crying because the champion will be back soon."

As we previously reported, Lesnar had been hospitalized due to a mysterious intestinal disorder. UFC president Dana White claimed the illness is so severe, Brock may never fight again.

So far, Brock has yet to release a comment himself.