British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a contest where the approaches never quite clicked, with frequent clinching marring the action.
Two judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," Buatsi commented in the ring as he was faced disapproval from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Various analysts believed Parker, 31, should have been awarded the win but the contender suffered the second setback of his 28-match career.
The Champion Secures Victory in Slow Affair
His career development has plateaued in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He began cautiously, with both competitors prepared to overcommit in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the cleaner periods.
The audience's tolerance diminished amid regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to secure the continental championship.
The arena became quiet for a extremely emotional homage before the fight as a bell ceremony sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound faded, the stillness transitioned to a chant of "only one Hatton".
The action during the bout matched the emotion. The eventual winner was knocked down by a significant blow directly to the chin in the early round, rising shakily to his feet on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising forming under his orbital area as the pair landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, 115-113 and 115-112, but both fighters were determined and courageous, exactly what their inspiration Hatton stood for.
