The 4-time world champion boxer claims he was training for a fight in March 2017, when he went to Dr. Gary Brazina about pain and clicking in his right elbow. He says the doc suggested arthroscopic surgery to fix it, and explained it would be relatively quick and painless, with plenty of time to recover for his fight.

According to the docs ... it didn't go as planned at all. Mosley says the surgery took hours longer than he was told, and Brazina revealed he had to "detach some ligaments and reattach them with screws."

Mosley claims he developed a fever and rash the next day, and Brazina failed to tell him he also suffered a severe burn during his surgery ... which became infected.

What's worse -- the boxer claims his doc altered some pre-op paperwork to make it look like Mosley consented to a more invasive surgery than he did.

He's suing Brazina and the hospital where his surgery was performed for damages. We reached out to Dr. Brazina ... so far, no word back.