Complex sentence: His boxing coach, Matthew Voltolini, adopted him as a son at age 12, and by 19, he had turned professional, putting together a 24-0 record in 6 years until his first loss came to Artur Grigorian in a WBO lightweight title match.
Split into simpler sentences: His boxing coach, Matthew Voltolini, adopted him as a son at age 12. By 19, Michael he had turned professional, with a 6-year 24-0 record, when his first loss came from Artur Grigorian in a WBO lightweight title match.