British light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive 10-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the styles didn't complement each other, with constant grabbing marring the contest.
The officials scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest showing but what defines champions? They win," Buatsi stated in the ring as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Various analysts felt Parker, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-match career.
The Champion Earns Victory in Dull Encounter
His career development has slowed in recent years through management issues and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter prepared to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the better periods.
The crowd's patience diminished during regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a outstanding all-Manchester battle to win the European light heavyweight title.
The stadium became quiet for a deeply poignant memorial before the fight as 10 bells rang in memory of the city's native Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the location since the passing of Hatton. As the last bell ended, the quiet became a shout of "there's only one Ricky Hatton".
The action inside the ropes equaled the emotion. Arthur was knocked down by a left hook squarely on the chin in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was injured soon, heavy swelling appearing under his orbital area as the duo traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were determined and courageous, all the qualities their idol Hatton represented.
