British light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding disrupting the contest.
The scoring judges had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They win," Buatsi said in the arena as he was met by boos from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the victory but the contender suffered the second setback of his 28-match professional journey.
Buatsi Earns Win in Slow Match
His career development has stalled in recent years through management issues and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to be aggressive in a calculated opening few rounds.
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 middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to barely take the cleaner periods.
The audience's tolerance wore thin amid frequent holding, with the challenger going down several times during close exchanges.
Parker closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Homage Inspires All-Manchester Fight
In the co-main event, Arthur overcame Rea by close verdict in a outstanding all-Manchester contest to capture the European light heavyweight title.
The venue became quiet for a profoundly moving homage ahead of the contest as a bell ceremony tolled in memory of Manchester's own Hatton, who died at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the last bell ended, the quiet became a shout of "only one Ricky Hatton".
The fighting during the bout matched the feeling. The eventual winner was knocked down by a left hook directly to the face in the initial stage, getting up unsteadily to his feet on the count of eight.
But the red-colored attire of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was damaged early, significant bruising developing under his face as the fighters traded powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the result with official scores of 114-114, 115-113 and wider gap, but both fighters were relentless and full of heart, exactly what their hero Hatton embodied.
