British light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a contest where the techniques failed to mesh, with constant grabbing spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best performance but what defines champions? They succeed," the victor stated in the ring as he was greeted with disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the win but the contender experienced the second defeat of his 28-bout professional journey.
Buatsi Claims Triumph in Slow Match
His career development has slowed in the past few years through contract problems and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the better rounds.
The audience's tolerance diminished amid frequent holding, with the challenger falling to the floor several times during grappling.
Parker ended stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester fight to win the continental title.
The venue fell silent for a deeply poignant homage prior to the bout as a bell ceremony sounded in honor of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the final chime faded, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The action during the bout matched the sentiment. Arthur was knocked down by a left hook directly to the chin in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was injured soon, heavy swelling appearing under his face as the duo exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur barely secured the result with judges scorecards of 114-114, narrow lead and 115-112, but each competitor were persistent and brave, exactly what their hero Hatton stood for.
