British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective individual punches in a contest where the approaches didn't complement each other, with constant grabbing disrupting the action.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," the victor commented in the ring as he was met by disapproval from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, 31, deserved the victory but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Earns Victory in Dull Match
His career development has plateaued in recent times through contract problems and considerable time of inactivity.
Once tipped as the future of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter willing to take risks in a tentative 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 making just his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to edge the cleaner periods.
The crowd's patience wore thin because of constant clinching, with the challenger falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a terrific local rivalry fight to secure the European light heavyweight championship.
The stadium grew hushed for a deeply poignant tribute ahead of the contest as 10 bells tolled in memory of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the city since Hatton's passing. As the concluding sound faded, the stillness transitioned to a shout of "there's just one Hatton".
The action during the bout matched the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the face in the early round, standing uncertainly to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was damaged early, noticeable damage developing under his eye 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 pace rarely dropped over the full distance.
The victor edged the verdict with judges scorecards of draw, narrow lead and wider gap, but both fighters were determined and courageous, exactly what their hero Hatton stood for.
