British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a close 10-round fight at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a match where the techniques never quite clicked, with frequent clinching spoiling the action.
The officials had it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but how do champions respond? They win," Buatsi commented in the ring as he was greeted with disapproval from the audience.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the victory but the contender suffered the second setback of his 28-match professional journey.
Buatsi Earns Triumph in Slow Affair
His career development has stalled in recent years through management issues and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight 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 fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to edge the better rounds.
The crowd's patience diminished because of constant clinching, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. However, former world champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Inspires Local Rivalry Fight
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester fight to capture the European light heavyweight title.
The venue became quiet for a deeply poignant tribute prior to the bout as a bell ceremony sounded in honor of local hero Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised fight night in the city since Hatton's passing. As the last bell disappeared, the stillness became a shout of "only one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was floored by a powerful punch squarely on the face in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was injured soon, significant bruising appearing under his face as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
The victor narrowly won the decision with official scores of draw, close margin and 115-112, but both fighters were persistent and brave, everything their idol Hatton represented.
