UK light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But Parker had his moments, delivering effective clean blows in a match where the styles didn't complement each other, with constant grabbing marring the contest.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They prevail," Buatsi said in the ringside as he was greeted with disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the victory but the contender endured the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Encounter
His career development has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, 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 opened tentatively, with neither fighter willing to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury 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 worked behind a solid defense to narrowly win the more effective stanzas.
The audience's tolerance wore thin because of constant clinching, with the challenger going down multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Homage Motivates Manchester Derby Contest
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester battle to win the continental championship.
The venue became quiet for a deeply poignant homage prior to the bout as a bell ceremony sounded in memory of local hero Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime disappeared, the quiet became a chant of "there's only one Hatton".
The competition during the bout matched the emotion. The eventual winner was knocked down by a left hook flush on the chin in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was damaged quickly, significant bruising appearing under his orbital area as the pair traded hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur narrowly won the result with official scores of 114-114, close margin and 115-112, but both fighters were determined and courageous, all the qualities their inspiration Hatton represented.
