British light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a contest where the approaches didn't complement each other, with excessive holding disrupting the contest.
The officials judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my finest display but how do champions respond? They prevail," the victor commented in the ringside as he was faced disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the victory but the British boxer suffered the second defeat of his 28-match career.
The Champion Secures Victory in Uneventful Affair
His career development has slowed in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors ready to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to narrowly win the better stanzas.
The crowd's patience decreased during frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker ended stronger, but several stanzas were hard to judge. However, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Fuels Manchester Derby Battle
In the co-main event, Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester contest to capture the European light-heavyweight championship.
The stadium fell silent for a profoundly moving homage ahead of the contest as a bell ceremony tolled in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the concluding sound ended, the stillness transitioned to a chant of "only one Ricky Hatton".
The competition within the ring matched the emotion. Arthur was knocked down by a significant blow directly to the jaw in the early round, rising shakily to his feet on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured early, significant bruising appearing under his face as the fighters traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor edged the verdict with judges scorecards of even score, 115-113 and clearer margin, but both fighters were determined and full of heart, all the qualities their inspiration Hatton stood for.
