British light-heavyweight Joshua Buatsi made a winning yet disappointing debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close 10-round fight at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a match where the approaches didn't complement each other, with frequent clinching spoiling the contest.
Two judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my best performance but what defines champions? They succeed," Buatsi commented in the arena as he was met by boos from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Various analysts felt Parker, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-fight professional journey.
Buatsi Secures Triumph in Dull Affair
Buatsi's progress has stalled in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with both competitors prepared to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to edge the cleaner stanzas.
The spectators' acceptance decreased during constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Arthur defeated Rea by split-decision in a terrific all-Manchester fight to capture the European light heavyweight belt.
The arena became quiet for a deeply poignant memorial before the fight as a bell ceremony tolled in honor of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound ended, the stillness became a chant of "there's only one Ricky Hatton".
The fighting within the ring equaled the emotion. Arthur was knocked down by a powerful punch directly to the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged early, noticeable damage developing under his eye as the duo landed powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur edged the result with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were relentless and courageous, all the qualities their inspiration the legend represented.
