British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the techniques failed to mesh, with excessive holding marring the action.
The scoring judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest display but what defines champions? They win," Buatsi stated in the ringside as he was greeted with disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
The Champion Secures Triumph in Uneventful Affair
His career development has slowed in recent years through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the better rounds.
The crowd's patience decreased during constant clinching, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Arthur overcame Rea by close verdict in a terrific local rivalry fight to secure the European light heavyweight belt.
The stadium became quiet for a profoundly moving homage prior to the bout as 10 bells tolled in honor of local hero Hatton, who succumbed aged 46 in recently.
It was the premier televised fight night in the city since the passing of Hatton. As the concluding sound disappeared, the stillness became a roar of "there's just one Hatton".
The fighting during the bout equaled the sentiment. Arthur was knocked down by a left hook squarely on the chin in the second round, rising shakily to his position on the referee's count.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured early, noticeable damage appearing under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of even score, narrow lead and clearer margin, but the two athletes were relentless and brave, everything their inspiration Hatton represented.
