UK light-heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a contest where the styles never quite clicked, with constant grabbing marring the action.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest showing but how do champions respond? They succeed," the victor commented in the ringside as he was faced boos from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts believed Parker, 31, should have been awarded the decision but the British boxer suffered the second setback of his 28-match career.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has stalled in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a tight guard to edge the more effective periods.
The crowd's patience wore thin during constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding local rivalry fight to secure the European light heavyweight belt.
The stadium fell silent for a extremely emotional tribute ahead of the contest as 10 bells sounded in memory of Manchester's own Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the stillness gave way to a chant of "there's just one Ricky Hatton".
The action during the bout matched the emotion. Arthur was knocked down by a left hook flush on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising developing under his face as the pair landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over the full distance.
Arthur barely secured the verdict with official scores of 114-114, narrow lead and 115-112, but the two athletes were determined and courageous, everything their inspiration Hatton embodied.
