UK light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight 10-round contest at the Manchester venue.
But the challenger found success, landing some telling single shots in a contest where the approaches never quite clicked, with excessive holding disrupting the fight.
The officials scored it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest display but what defines champions? They win," the victor commented in the arena as he was faced boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts thought Parker, thirty-one, should have been awarded the victory but the contender experienced the second defeat of his 28-match professional journey.
Buatsi Earns Win in Dull Match
His career development has stalled in recent years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to realize his potential - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors ready to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who utilized a protective stance to barely take the better stanzas.
The crowd's patience decreased amid regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Contest
In the co-main event, Arthur beat Rea by divided decision in a outstanding local rivalry fight to capture the continental belt.
The arena grew hushed for a extremely emotional memorial prior to the bout as a bell ceremony rang in honor of the city's native Hatton, who succumbed 46 years old in recently.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The fighting during the bout reflected the sentiment. Arthur was floored by a powerful punch directly to the jaw in the early round, rising shakily to his position on the referee's count.
But the traditional red attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling forming under his eye as the fighters landed hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
The victor barely secured the decision with judges scorecards of even score, 115-113 and 115-112, but both fighters were persistent and brave, everything their idol the legend stood for.
