The fight ending sequence was a bad break (no pun intended) for Trejo, who was otherwise handling the younger and less famous of the fighting Pacquiao's heading into the round.

The injury came when Trejo caught an errant left hand on his right forearm about a minute into the 4 th round while looking to throw a right of his own. Trejo dropped his guard, grabbing his right arm in pain as Pacquiao nailed him with a flush left to the chin. Trejo took a knee and immediately bounced up, frantically walking to his corner to inform his handlers of the injury. The referee was left with no choice but to wave off the action.

From another site.