British light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with constant grabbing spoiling the fight.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best performance but what defines champions? They prevail," Buatsi commented in the ringside as he was greeted with boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers thought Parker, thirty-one, merited the win but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
The Champion Secures Triumph in Dull Encounter
His career development has stalled in recent years through management issues and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who utilized a solid defense to barely take the more effective periods.
The spectators' acceptance wore thin amid regular grabbing, with Parker going down various instances during close exchanges.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester contest to capture the European light heavyweight championship.
The stadium grew hushed for a deeply poignant memorial prior to the bout as 10 bells tolled in memory of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised match card in the location since the passing of Hatton. As the final chime faded, the stillness gave way to a roar of "only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a powerful punch flush on the face in the initial stage, standing uncertainly to his position on the count of eight.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling appearing under his face as the fighters landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur edged the decision with scoring tallies of 114-114, close margin and clearer margin, but each competitor were relentless and courageous, all the qualities their idol the legend stood for.
