UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round contest at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a match where the styles never quite clicked, with excessive holding marring the action.
Two judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts felt the challenger, thirty-one, deserved the decision but the Derbyshire fighter experienced the second setback of his 28-fight career.
Buatsi Secures Triumph in Slow Encounter
Buatsi's progress has stalled in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to fulfil that early promise - 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 single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to barely take the cleaner periods.
The crowd's patience wore thin during frequent holding, with the challenger going down various instances during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Arthur overcame Rea by split-decision in a terrific all-Manchester battle to secure the continental championship.
The venue became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the premier televised fight night in the city since Hatton's passing. As the final chime ended, the stillness transitioned to a chant of "only one Ricky Hatton".
The action during the bout reflected the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the face in the early round, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up quickly, noticeable damage forming under his face as the pair landed hard punches.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, close margin and 115-112, but each competitor were determined and brave, exactly what their hero Hatton embodied.
