British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a fight where the techniques didn't complement each other, with excessive holding marring the fight.
Two judges scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," Buatsi said in the arena as he was greeted with boos from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, merited the decision but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
Buatsi Secures Victory in Dull Affair
His career development has slowed in recent times through promotional disputes and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers willing to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super middleweight 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 halfway stage, the underdog had kept it close, creating difficulties for the favorite, who utilized a tight guard to barely take the better periods.
The crowd's patience diminished because of constant clinching, with the challenger falling to the floor various instances during grappling.
The underdog ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Arthur beat Rea by close verdict in a outstanding Manchester vs Manchester fight to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in remembrance of the city's native Hatton, who passed away 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound ended, the silence transitioned to a roar of "only one Hatton".
The action within the ring matched the sentiment. Arthur was sent to the canvas by a left hook flush on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored shorts of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up soon, noticeable damage forming under his orbital area as the fighters exchanged hard punches.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the result with judges scorecards of 114-114, 115-113 and 115-112, but the two athletes were determined and brave, everything their inspiration Hatton represented.
