British light heavyweight Joshua Buatsi earned a win yet disappointing debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the techniques didn't complement each other, with constant grabbing marring the fight.
The officials had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but what do champions do? They prevail," Buatsi stated in the ring as he was faced disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, thirty-one, deserved the victory but the contender suffered the second loss of his 28-fight professional journey.
The Champion Claims Win in Slow Match
The fighter's advancement has plateaued in recent times through management issues and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter ready to overcommit in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who worked behind a tight guard to barely take the cleaner periods.
The crowd's patience diminished because of frequent holding, with the challenger going down several times during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester contest to secure the continental belt.
The stadium fell silent for a deeply poignant memorial ahead of the contest as a bell ceremony sounded in memory 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 last bell faded, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The action within the ring reflected the feeling. The eventual winner was sent to the canvas by a left hook flush on the jaw in the second round, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was injured early, significant bruising developing under his orbital area as the fighters traded hard punches.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
Arthur barely secured the decision with scoring tallies of even score, 115-113 and wider gap, but each competitor were relentless and brave, exactly what their idol Hatton embodied.
