Already understrength after losing Jeremy Marshall-King and Felise Kaufusi to suspension and Isaiya Katoa to an elbow injury pre-game, the Dolphins were decimated in the 'Battle For Brisbane', losing a trio of ex-Broncos.
The NRL's newest club were valiant despite losing troops, only trailing 14-12 before a runaway Kotoni Staggs try in the 79th minute put the inaugural clash to bed, screaming 'our f***ing home' to the Broncos' crowd after scoring the match-winner.
However, the Dolphins now look set to be without their halfback for months, with fears O'Sullivan suffered a torn pec 61 minutes into the match, ending the game in a sling as he readies himself for a long sideline stint.
The Dolphins confirmed the halfback would require surgery on the torn pec, leaving the journeyman sidelined for 3-4 months.
Wayne Bennett confirmed the blow.
“It's a shame because Sean has been great for us in the opening month,” Bennett said in the post-match press conference.
“Sean is a guy who will never let you down, he has been great value for us, he has done a very good job steering us around.”
It leaves O'Sullivan racing the clock to face Brisbane in the rematch held at the Gabba in Round 18, just over three months from the injury.
It opens the door for Anthony Milford, a late call-up for Katoa, to remain in the side for the coming months, and push Katoa to halfback when he returns.
The other halfback option is another casualty from the clash. Kodi Nikorima, currently wearing the No. 9 in Marshall-King's absence, lasted just two minutes of the match before copping a heavy head knock, splitting the utility open.
Despite passing his HIA, Nikorima needed too many stitches to return to play. Mason Teague and Ray Stone split the hooking duties, however Nikorima should be right to go for next week's clash against St George Illawarra.
Lastly came Tesi Niu, who bravely played the entire 80 minutes despite copping a leg injury just minutes into the contest.
The former Bronco scored a hat-trick last week against Newcastle, the club's first-ever, however was on one leg for the vast majority of the local derby.
The Dolphins will get Jeremy Marshall-King back from suspension next weekend while Isaiya Katoa will be close to returning from his elbow niggle, with the 19-year-old coming within whiskers of playing Friday night.