British light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a contest where the techniques didn't complement each other, with frequent clinching disrupting the action.
The officials scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best display but what defines champions? They succeed," the victor said in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
Buatsi Secures Victory in Slow Match
Buatsi's progress has plateaued in recent years through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a tentative early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second bout at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to narrowly win the more effective stanzas.
The crowd's patience diminished amid constant clinching, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry battle to secure the European light heavyweight belt.
The arena grew hushed for a deeply poignant memorial before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime disappeared, the silence transitioned to a chant of "only one Hatton".
The competition within the ring equaled the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was marked up quickly, noticeable damage appearing under his face as the pair landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the verdict with judges scorecards of 114-114, close margin and 115-112, but the two athletes were relentless and full of heart, exactly what their hero the legend embodied.
