The young South Korean was making his first appearance since February 11 and fluffed a simple close-range chance that ultimately proved costly for The Black Cats
Ji, making his first appearance since February 11, had been on the field only a matter of minutes before he missed a simple close-range chance after James McClean's low cross with the score at 1-0.
Sunderland are 6/1 with William Hill to beat QPR 1-0 at home on Saturday.
Yakubu Aiyegbeni scored on 86 minutes to seal the win for Rovers but O'Neill opted not to berate the young substitute despite his miss.
"It was a good chance, we missed that chance," O'Neill told BBC Sport.
"The young lad Ji wasn't long on the pitch so that's probably an excuse for that. That was unfortunate, that would've made it one each."
Speaking later to the club's website, O'Neill added: "We missed a great opportunity to equalise when the ball fell to young Ji but we should have been able to create more throughout the game."
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