British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive 10-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the styles failed to mesh, with frequent clinching marring the contest.
The officials judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They win," Buatsi commented in the ring as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, 31, merited the win but the British boxer suffered the second defeat of his 28-fight professional journey.
Buatsi Secures Win in Uneventful Encounter
The fighter's advancement has slowed in recent times through contract problems and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with both competitors ready to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to barely take the better periods.
The audience's tolerance diminished amid regular grabbing, with Parker going down several times during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry battle to capture the European light-heavyweight title.
The arena became quiet for a deeply poignant homage before the fight as ten bells rang in honor of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's death. As the final chime disappeared, the silence transitioned to a shout of "there's only one Hatton".
The action within the ring equaled the emotion. Arthur was sent to the canvas by a significant blow squarely on the chin in the second round, rising shakily to his position on the referee's count.
But the traditional red shorts of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was injured quickly, heavy swelling forming under his orbital area as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the decision with judges scorecards of draw, close margin and clearer margin, but each competitor were determined and brave, exactly what their inspiration Hatton stood for.
