UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a contest where the approaches failed to mesh, with frequent clinching marring the contest.
The scoring judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi stated in the ring as he was faced disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt Parker, 31, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-match career.
The Champion Claims Victory in Dull Match
The fighter's advancement has stalled in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a last chance - under recent support.
He began cautiously, with both competitors ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who worked behind a solid defense to narrowly win the cleaner periods.
The audience's tolerance wore thin amid regular grabbing, with Parker going down several times during close exchanges.
The underdog closed well, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester fight to secure the European light heavyweight belt.
The arena fell silent for a extremely emotional homage prior to the bout as 10 bells sounded in memory of local hero Hatton, who succumbed at age 46 in recently.
It was the premier televised match card in the city since the passing of Hatton. As the final chime faded, the silence transitioned to a roar of "there's only one Ricky Hatton".
The competition during the bout reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch flush on the jaw in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling forming under his eye as the pair landed powerful shots.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the decision with official scores of 114-114, 115-113 and wider gap, but the two athletes were persistent and brave, all the qualities their hero the legend represented.
