British light-heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques failed to mesh, with excessive holding disrupting the fight.
The officials had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the ring as he was faced disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, 31, merited the win but the contender suffered the second loss of his 28-fight career.
The Champion Claims Victory in Uneventful Match
His career development has slowed in recent years through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a calculated early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a protective stance to edge the more effective stanzas.
The spectators' acceptance diminished during constant clinching, with Parker slipping to the canvas several times during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Although, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the continental belt.
The stadium became quiet for a profoundly moving tribute ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who died aged 46 in September.
It was the first major televised boxing event in the location since Hatton's death. As the concluding sound disappeared, the silence gave way to a chant of "there's only one Hatton".
The competition during the bout matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the jaw in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was injured quickly, heavy swelling forming under his orbital area as the pair exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the decision with scoring tallies of even score, narrow lead and clearer margin, but each competitor were persistent and full of heart, all the qualities their idol the legend represented.
