Mammadov made the better start and took the first round 9-4, but was pegged back slighty in the second.
The referee then gave the Azeri a two-point warning for persistent holding, which gave Karneyeu two points to his total.
Another warning for the same reason came Mammadov's way in the third, at a time when he was barely able to stand up due to exhaustion.
An uninspiring bout was brought to a close with the Belorusian finishing strongly.
However, Mammadov won through on countback after the judges had scored the fight even at 19-19.