Judge Maximo de Luca scored the bout 115-112, John Keane scored it 116-112 and Jack Woodburn saw it 116-111 for Pascal, who improved to 23-1 with 15 knockouts.
It was Pascal’s first fight since stepping up to light heavyweight. He lost a bid for the WBC super-middleweight belt to Carl Froch on December 6 in Nottingham, England.
He didn’t disappoint, downing Diaconu with a left in the fifth and unleashing another flurry after the champion rose.
Diaconu was able to withstand the onslaught and finished the round with a burst of his own.
In the 11th round, Diaconu rocked Pascal with a right to the jaw and the two fighters went toe-to-toe in an exchange that brought the crowd to its feet.
There was more action in the 12th, when a bloodied Diaconu was unable to put Pascal down.