Erasmus sniped the spoils after Mbulelo Mabizela and Lehlohonolo Majoro both netted against their former teams at Moses Mabhida Stadium.
Ex-Bucs captain Mabizela bagged the opener just past the hour-mark on his Usuthu debut, having been sacked by Mpumalanga Black Aces in December after falling out with head coach Clive Barker.
Majoro, however, equalised in the 71st minute before Erasmus struck at the death to ensure victory for the Buccaneers on their return to League action.
Not being able to maintain a lead continues to be AmaZulu’s weakness as they remain last on the log, whilst Pirates maintain their fourth spot behind Bidvest Wits.
AmaZulu came to this match desperate for a win and they showed no respect to Pirates as they dominated the first half.
Usuthu continued to press after the break, with Mabizela breaking the deadlock in the 66th minute when he headed in a Marc van Heerden free-kick.
Majoro, though, hit back with a lob soon after before teeing up Erasmus for the decisive blow of the contest.