British light heavyweight Joshua Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close 10-round bout at the boxing arena.
But Parker had his moments, landing some telling single shots in a match where the approaches failed to mesh, with frequent clinching spoiling the fight.
The officials judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best display but how do champions respond? They win," the victor commented in the arena as he was faced boos from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Various analysts thought the challenger, 31, deserved the decision but the contender suffered the second setback of his 28-bout career.
The Champion Earns Win in Slow Match
Buatsi's progress has plateaued in recent times through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who worked behind a protective stance to barely take the more effective rounds.
The audience's tolerance diminished during constant clinching, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but several stanzas were difficult to score. Although, former world champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Inspires Manchester Derby Battle
In the co-main event, Arthur defeated Rea by split-decision in a terrific all-Manchester battle to secure the European light heavyweight championship.
The arena fell silent for a deeply poignant homage ahead of the contest as a bell ceremony tolled in memory of local hero Ricky Hatton, who died at age 46 in last month.
It was the premier televised match card in the city since Hatton's passing. As the final chime disappeared, the stillness gave way to a roar of "only one Hatton".
The action within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was injured quickly, significant bruising forming under his face as the fighters landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the decision with judges scorecards of even score, narrow lead and clearer margin, but each competitor were relentless and courageous, exactly what their hero Hatton stood for.
