British light-heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful single shots in a contest where the approaches didn't complement each other, with constant grabbing marring the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor said in the ringside as he was met by negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the win but the Derbyshire fighter experienced the second setback of his 28-match career.
Buatsi Earns Triumph in Slow Encounter
Buatsi's progress has stalled in the past few years through contract problems and considerable time of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter prepared to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the cleaner rounds.
The crowd's patience wore thin because of regular grabbing, with the challenger falling to the floor various instances during clinches.
Parker ended stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Arthur overcame Rea by divided decision in a excellent local rivalry contest to win the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound faded, the silence transitioned to a roar of "there's only one Ricky Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a left hook flush on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured early, heavy swelling forming under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of 114-114, narrow lead and clearer margin, but the two athletes were relentless and courageous, exactly what their idol the legend embodied.
