UK light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the approaches never quite clicked, with constant grabbing disrupting the contest.
The officials had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi said in the arena as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Secures Win in Dull Encounter
The fighter's advancement has stalled in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a last chance - under fresh management.
He began cautiously, with both competitors prepared to be aggressive in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who utilized a protective stance to barely take the cleaner rounds.
The crowd's patience diminished because of regular grabbing, with the challenger slipping to the canvas various instances during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the continental title.
The arena fell silent for a profoundly moving homage before the fight as a bell ceremony sounded in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The fighting during the bout reflected the emotion. Arthur was sent to the canvas by a powerful punch directly to the jaw in the early round, rising shakily to his footing on the referee's count.
But the red-colored shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged quickly, heavy swelling forming under his orbital area as the fighters exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur barely secured the decision with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were relentless and full of heart, everything their hero Hatton represented.
