UK light-heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive 10-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a match where the approaches never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with boos from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, 31, merited the win but the Derbyshire fighter experienced the second setback of his 28-match career.
The Champion Claims Victory in Dull Encounter
His career development has slowed in recent years through management issues and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to edge the better periods.
The crowd's patience decreased amid frequent holding, with Parker slipping to the canvas various instances during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester fight to secure the European light heavyweight championship.
The venue grew hushed for a deeply poignant memorial prior to the bout as ten bells tolled in memory of the city's native Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the last bell faded, the stillness transitioned to a chant of "there's just one Hatton".
The competition within the ring equaled the emotion. The eventual winner 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 Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising appearing under his orbital area as the duo landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the action hardly diminished over 12 rounds.
The victor narrowly won the result with official scores of even score, 115-113 and clearer margin, but both fighters were determined and brave, exactly what their hero Hatton embodied.
