Wayne Bennett admits he got it wrong moving Adam Doueihi to the wing alongside James Roberts as South Sydney's were thrashed 30-6 by the Sydney Roosters.
Having started on the wing only once in his NRL career before Friday night's qualifying final, Bennett shifted Doueihi from fullback to the edge before kick off.
Roberts was also in foreign territory inside him on the left edge, having played on the right for the large majority of his NRL and State of Origin career.
The plan backfired, as the Tricolours ran in three first half tries down their edge to knock the Rabbitohs out of the match early.
"I made a mistake tonight, putting Adam Doueihi on the wing there," Bennett said.
"He hasn't played there before but he's played with such confidence in every position I have put him with all the injuries.
"I thought he would handle that but maybe it was a bridge too far for him at this stage of his footy career."
Bennett is likely to make changes next week, with Dane Gagai set to return from a calf injury next Friday night against the loser of Cronulla and Manly.
The most likely reaction will be Gagai going back on the left centre with Roberts on the right, and Campbell Graham returning to the wing.
Roberts had played right centre since shifting to Souths before his needless ban three weeks ago for late contact on Brisbane's Corey Oates.
But he was used to plug the left-edge hole created by Gagai's hamstring injury on Friday night, only to also be troubled by Roosters centre Joseph Manu.
A week after doing a number on Gagai, Manu got the Roosters' first try in just the second minute when he hit and spun through Roberts.
Eight minutes later Roberts was caught out again, sprinting in field out of marker before Cooper Cronk sent it back down his edge on the last as the Roosters found Souths short.
The ball when through five sets of hands before Mitchell Aubusson dived over, putting them up 10-0 after 11 minutes
Manu then got the better of Roberts again for their third down that side.
Roberts also finished the game with two second-half errors, kicked the ball out dead on the full once and failed to bust a tackle in the loss.