UK light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a match where the approaches didn't complement each other, with frequent clinching spoiling the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi said in the ring as he was greeted with boos from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers felt Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
The Champion Claims Victory in Dull Match
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to edge the better rounds.
The crowd's patience decreased during regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Arthur overcame Rea by close verdict in a outstanding local rivalry battle to secure the continental championship.
The stadium became quiet for a deeply poignant homage prior to the bout as a bell ceremony rang in honor of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell disappeared, the stillness became a shout of "there's just one Hatton".
The action during the bout matched the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the second round, rising shakily to his footing on the eight count.
But the traditional red shorts of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was damaged soon, heavy swelling developing under his face as the pair landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor barely secured the result with official scores of even score, narrow lead and 115-112, but both fighters were determined and courageous, exactly what their idol Hatton embodied.
