UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling single shots in a contest where the styles never quite clicked, with constant grabbing marring the fight.
The officials scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the ringside as he was met by boos from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts believed Parker, 31, deserved the win but the British boxer experienced the second setback of his 28-match career.
Buatsi Claims Victory in Dull Encounter
Buatsi's progress has slowed in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to barely take the more effective periods.
The audience's tolerance diminished amid constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific all-Manchester contest to win the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving tribute prior to the bout as a bell ceremony sounded in honor of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the last bell disappeared, the stillness transitioned to a roar of "only one Hatton".
The fighting during the bout matched the sentiment. The eventual winner was knocked down by a significant blow flush on the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, significant bruising appearing under his eye as the pair landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the decision with official scores of 114-114, close margin and 115-112, but both fighters were determined and brave, exactly what their hero the legend embodied.
