UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive ten-round fight at the boxing arena.
But Parker found success, landing some telling individual punches in a match where the approaches never quite clicked, with excessive holding disrupting the contest.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," the victor stated in the arena as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Several boxing experts felt Parker, 31, merited the win but the British boxer suffered the second setback of his 28-bout career.
The Champion Earns Win in Dull Match
Buatsi's progress has slowed in recent times through contract problems and considerable time of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who utilized a protective stance to edge the cleaner rounds.
The audience's tolerance decreased during regular grabbing, with Parker falling to the floor multiple occasions during clinches.
Parker closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a terrific local rivalry battle to capture the European light-heavyweight belt.
The stadium fell silent for a deeply poignant tribute before the fight as a bell ceremony sounded in remembrance of the city's native Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the urban center since Hatton's passing. As the concluding sound ended, the stillness gave way to a chant of "only one Hatton".
The fighting within the ring reflected the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, getting up unsteadily to his footing on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up soon, noticeable damage appearing under his face as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the verdict with official scores of draw, close margin and 115-112, but the two athletes were persistent and courageous, exactly what their inspiration Hatton represented.
