My gut feeling says Fenech. He was in so many tough fights, I think he pulls this off. It's not like Mares is a pop and move type of boxer, so his style would play into Fenech's hands.
If Abner gets Russell Mora, he can low blow Jeff into Boliivan. Otherwise, I don't think Abner can handle Jeff's strength. Jeff used to muscle around rugby players in the gym.
Mares doesn't have the crunch in his punches to stop or keep Fenech at bay.Mares surely can't stand Fenech's all out assault round after round. Jeffrey via TKO 11.