Inoue stops Donaire in second round to unify bantamweight titles
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Inoue, who also beat Donaire in a 2019 classic, floored the 39-year-old veteran at the end of the first round and stopped him 1 min, 24 sec into the second to take his record to 23 wins, 20 of them by knockout.
Inoue and Donaire's previous bout in Saitama, north of Tokyo, was voted fight of the year by the Boxing Writers Association of America, with the Japanese fighter recovering from a fractured eye socket and double vision to claim a unanimous points win.
The hotly anticipated rematch at the same venue turned out to be a completely different contest, with Inoue living up to his "Monster" nickname by dominating the Philippines veteran.
He landed a heavy shot that wobbled Donaire on his feet, before a barrage of punches sent the "Filipino Flash" to the floor and convinced the referee to stop the fight.
He has said he wants to become the undisputed champion before the end of the year.
Butler captured previous champion John Riel Casimero's vacant WBO title when he beat Jonas Sultan by unanimous decision in April.
Butler was elevated from interim to full champion last month when the WBO stripped Casimero of the full title.