Marvellous Marvin, now a resident in northern Italy, was at ringside in Milan on Saturday to give the seal of approval to Reid's dismantling of the 30-year-old former Rome policeman.
A switch to southpaw in round seven hurried the 25-year-old Reid's triumph, and Hagler, the former undisputed world middleweight champion, told him: "You did it my way, switched and won it with a body shot."
Reid said: "But we all knew I could do it. Some people said I didn't deserve the chance because I hadn't fought for a British or European title."
Nardiello, who sustained two cracked bones in his left hand and will be in plaster for a month, said: "Fighting in these circumstances was impossible.
"I broke my hand in the first round, I tried an uppercut and hit Reid on the head. I felt bad, I told my manager but he didn't believe me, so I gritted my teeth for six rounds."Reuse content