Above, a cover for Kid Galahad by Francis Wallace. You get all the elements here—the natural talent, the meteoric rise from obscurity, the weakness for women, the predatory gangsters, the big fixed fight, etc. The book is originally copyright 1936 with this Bantam edition fronted by Charles Andres art appearing in 1947.
Okay, Galahad—I want you to forget you're the most gallant of all the knights and beat this guy like a circus monkey.