British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight 10-round fight at the boxing arena.
But Parker found success, delivering effective single shots in a match where the techniques didn't complement each other, with frequent clinching marring the fight.
The officials had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," Buatsi commented in the arena as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the contender suffered the second setback of his 28-fight professional journey.
The Champion Earns Win in Slow Encounter
His career development has slowed in recent years through management issues and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a last chance - under recent support.
He began cautiously, with the two boxers ready to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for Buatsi, who utilized a protective stance to edge the more effective rounds.
The crowd's patience wore thin amid constant clinching, with the challenger going down various instances during clinches.
Parker finished the stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester fight to win the European light heavyweight championship.
The arena grew hushed for a profoundly moving tribute prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the city since Hatton's passing. As the last bell ended, the quiet gave way to a chant of "there's only one Hatton".
The action inside the ropes reflected the emotion. Arthur was floored by a powerful punch squarely on the jaw in the second round, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was damaged quickly, significant bruising forming under his face as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the decision with official scores of even score, close margin and wider gap, but both fighters were relentless and brave, all the qualities their idol Hatton represented.
