British light heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a competitive 10-round contest at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches never quite clicked, with frequent clinching disrupting the action.
Two judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest display but what defines champions? They prevail," Buatsi commented in the ring as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts thought Parker, 31, merited the win but the British boxer experienced the second loss of his 28-bout professional journey.
Buatsi Secures Win in Dull Encounter
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter prepared to take risks in a cagey initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to narrowly win the more effective periods.
The crowd's patience decreased amid frequent holding, with the challenger falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to capture the continental title.
The stadium became quiet for a profoundly moving homage prior to the bout as a bell ceremony tolled in honor of the city's native Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound disappeared, the stillness gave way to a roar of "only one Ricky Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the jaw in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was damaged soon, noticeable damage appearing under his eye as the duo landed powerful shots.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the verdict with official scores of draw, close margin and 115-112, but the two athletes were relentless and full of heart, exactly what their hero Hatton stood for.
