Lomachenko has already fought several six rounders with no headgear against good comp. He fought in that semi-pro boxing league.
Vinnie, please don't forget the condition Vysotskyy laid out as the OP: I'd already reiterated Vysostskyy's restriction in post #15: Not picking on you [certainly not, as you're one of my favorite posters], just reemphasizing what an excellent challenge the thread creator laid out for us.
The massive Scottish flyweight Jackie Paterson knocked out 29-1 (and future flyweight world champion) Rinty Monaghan (in 5 rounds no less) in just his second professional bout. I just wanted to mention it.
Evander Holyfield had a very good and tough first fight against a boxer by the name of Lionel Byarm. Byarm by no means was a great fighter but was a solid pro and gave the real deal a very tough test in his first fight.