I think Mancinni only had 40 amature fights, and at his best he went 14 rounds with a near prime Arguello!
The amateur game is totally different than the pro game and the amateurs ... the amateurs is a strong emphasis on fundamentals ... throwing straight punches ... ducking, slipping, countering, leading, footwork, stance, and balance. You can learn all of that in the pros ... there are tons of fighters that have little to none amateur experience and turn out to be world champions.
I think if you're going to have little to no amateur career you need a huge natural asset like tons of power or extreme speed.
Add to that B-Slice ... Yuriorkis Gamboa 2/3 of the 2000 US olympic team Zahir Raheem Mohammad Abdulaev Oktay Urkal
Depends you can go the SRL or DLH way or the Howard Davis way....I do prefer fighters with stellar amateur backgrounds who have proven himself at the pro level .. I like fighters like Gamboa and Donaire over Guzman and Mijares...thats just based on a comparative analysis rate of success. When you hear a Howard Davis..you scratch your head..not the other way around
Having an amateur career certainly gives guys a leg up, but it is no guarantee. There are many great ams who flopped in the pros (Mark Breland comes to mind), and many guys with little or no amateur fights who became ATGs. Saying amateur background is necessary for pro success is an unwarrented over-generalization.