UK light-heavyweight Joshua Buatsi made a winning yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round fight at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a fight where the styles failed to mesh, with constant grabbing marring the contest.
The officials scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest performance but how do champions respond? They succeed," Buatsi stated in the arena as he was faced disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Various analysts believed Parker, 31, deserved the decision but the British boxer endured the second setback of his 28-match career.
Buatsi Earns Victory in Uneventful Encounter
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to barely take the more effective rounds.
The audience's tolerance decreased amid constant clinching, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a outstanding all-Manchester battle to capture the continental belt.
The venue fell silent for a deeply poignant memorial before the fight as ten bells sounded in remembrance of local hero Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound ended, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The fighting during the bout reflected the emotion. The eventual winner was knocked down by a left hook directly to the face in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged quickly, significant bruising forming under his orbital area as the fighters exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of draw, close margin and 115-112, but the two athletes were relentless and courageous, all the qualities their hero Hatton embodied.
