For about 4 years, Sylvester Stallone managed the boxing career of Lee Canalito, even letting him use the nickname "Italian Stallion".
Sly was convinced Canalito was a huge star in the making. He cast Canalito as Victor Carboni in Paradise Alley, taking the less flattering role of con man brother Cosmo Carboni for himself.
Victor's character was kind of cross between Rocky and Lenny from Of Mice and Men (if Lenny lived in Hell's Kitchen in the late 1940s).
The "Of Mice and Men" similarity was not accidental. Sly loved its deceptive simplicity. He was a big fan of the 1939 film adaptation with Burgess Meredith in the George role. It was also not accidental that one of the Carboni brothers is named Lenny (although he's not the simple-minded and sweet but freakishly strong character).
Personally, i think part of why Paradise Alley failed -- it's a very underrated movie IMO -- is that audiences wanted and expected Sly to be the more Rocky-like character, not a selfish and exploitive (but mostly inept) hustler who is pretty hard to like until near the end of the movie when he finds his conscience just as embittered wounded WW2 veteran brother Lenny loses his.
At any rate, Sly invested time and effort into dabbling as a boxing manager in addition to acting (Nighthawks, Victory, Rocky 3).
Something about Sly: he's very daring and ambitious -- writing, acting, directing, producing, painting, dabbling in "real life boxing" management, even trying to sing (Paradise Alley theme song -- worst part of the otherwise underrated movie-- and the awful Rhinestone movie opposite Dolly Parton). He might fail, and sometimes has spectacularly, but he'll give it his all regardless of what skeptics and critics think.
Back to Paradise Alley for a second, one critic wrote that Sly was just "trying to redo Rocky, except with a different hat, an earring and wrestling ring."
He got no credit for writing Paradise Alley as a novel before it was a screenplay; two very different types of writing challenges.
As the writer/director/star, he tackled a very different character than Rocky for his role. Thematically, he showed respect to the pro wrestling business when most everyone else treated it as joke. That latter aspect also turned off critics of the time.
As for the singing , well, he should have listened to his Rocky alter ego ("I can't sing or dance"). If he wanted a Stallone to sing "Too Close to Paradise" (which Bill Conti co-wrote, by the way), he should have tabbed Frank for that instead.
@mikesielski