British light heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close ten-round fight at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a contest where the styles didn't complement each other, with excessive holding disrupting the fight.
Two judges judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," Buatsi stated in the arena as he was met by negative reactions from the crowd.
The Londoner 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 thought Parker, 31, should have been awarded the win but the British boxer suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Win in Dull Match
His career development has stalled in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - possibly in what seems like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to edge the better rounds.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
The underdog closed well, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a excellent all-Manchester contest to win the European light heavyweight championship.
The arena grew hushed for a profoundly moving homage ahead of the contest as 10 bells tolled in honor of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the concluding sound faded, the quiet transitioned to a roar of "only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a powerful punch directly to the chin in the second round, rising shakily to his footing on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged quickly, significant bruising forming under his face as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the decision with judges scorecards of 114-114, 115-113 and 115-112, but the two athletes were relentless and courageous, everything their inspiration Hatton stood for.
