UK light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a fight where the styles failed to mesh, with constant grabbing spoiling the action.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They prevail," the victor stated in the arena as he was greeted with boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-bout career.
The Champion Claims Victory in Dull Encounter
His career development has plateaued in the past few years through management issues and extended periods of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter ready to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who utilized a protective stance to barely take the more effective rounds.
The audience's tolerance diminished amid regular grabbing, with Parker slipping to the canvas several times during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester fight to capture the continental title.
The venue fell silent for a profoundly moving memorial ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound ended, the quiet transitioned to a shout of "only one Ricky Hatton".
The fighting within the ring reflected the feeling. The eventual winner was knocked down by a powerful punch directly to the chin in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising forming under his orbital area as the pair landed hard punches.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of even score, narrow lead and wider gap, but both fighters were persistent and full of heart, all the qualities their inspiration the legend stood for.
