British light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a contest where the styles never quite clicked, with excessive holding spoiling the fight.
The scoring judges scored it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but what defines champions? They prevail," Buatsi said in the ring as he was faced negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers felt Parker, 31, merited the decision but the contender endured the second setback of his 28-fight professional journey.
The Champion Secures Win in Uneventful Encounter
His career development has stalled in recent years through management issues and extended periods of lack of fights.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who worked behind a protective stance to barely take the cleaner rounds.
The spectators' acceptance decreased because of constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
Parker closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific local rivalry contest to capture the continental title.
The arena fell silent for a deeply poignant homage before the fight as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the final chime ended, the silence became a chant of "there's just one Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was knocked down by a significant blow directly to the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was injured quickly, significant bruising appearing under his eye as the pair exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
The victor barely secured the result with scoring tallies of draw, close margin and 115-112, but each competitor were determined and courageous, everything their inspiration Hatton stood for.
