UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight 10-round fight at the boxing arena.
But Parker had his moments, landing some telling individual punches in a match where the techniques never quite clicked, with frequent clinching marring the action.
The scoring judges scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi commented in the ring as he was faced boos from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Various analysts felt the challenger, 31, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-match career.
Buatsi Earns Win in Uneventful Encounter
Buatsi's progress has stalled in recent times through management issues and considerable time of lack of fights.
Once tipped as the future of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to barely take the better stanzas.
The spectators' acceptance decreased amid regular grabbing, with Parker going down various instances during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Although, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Arthur defeated Rea by divided decision in a excellent all-Manchester contest to secure the European light heavyweight title.
The arena became quiet for a deeply poignant homage ahead of the contest as ten bells rang in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime ended, the quiet gave way to a chant of "there's only one Ricky Hatton".
The action during the bout matched the emotion. Arthur was floored by a significant blow directly to the chin in the early round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged soon, significant bruising forming under his eye as the fighters exchanged hard punches.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
The victor barely secured the verdict with scoring tallies of even score, close margin and clearer margin, but each competitor were persistent and courageous, all the qualities their inspiration Hatton embodied.
