British light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a match where the approaches failed to mesh, with frequent clinching spoiling the contest.
Two judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best display but how do champions respond? They win," Buatsi commented in the ring as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought the challenger, 31, deserved the win but the British boxer suffered the second defeat of his 28-match professional journey.
Buatsi Earns Win in Uneventful Affair
His career development has plateaued in recent times through contract problems and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - maybe in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers willing to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to narrowly win the cleaner periods.
The crowd's patience decreased during frequent holding, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Homage Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester fight to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who died 46 years old in recently.
It was the first major televised fight night in the location since Hatton's passing. As the final chime ended, the quiet transitioned to a chant of "there's only one Hatton".
The action during the bout equaled the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was injured early, noticeable damage developing under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor edged the decision with official scores of draw, close margin and 115-112, but the two athletes were determined and courageous, all the qualities their hero the legend represented.
