UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling single shots in a fight where the approaches failed to mesh, with excessive holding marring the fight.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They win," the victor stated in the ring as he was met by negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, should have been awarded the win but the British boxer experienced the second loss of his 28-match career.
The Champion Secures Win in Uneventful Affair
His career development has stalled in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors prepared to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second fight at the higher weight class.
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 cleaner rounds.
The spectators' acceptance wore thin during frequent holding, with Parker going down various instances during grappling.
The underdog ended stronger, but multiple periods were difficult to score. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to secure the continental championship.
The venue grew hushed for a extremely emotional homage ahead of the contest as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the urban center since the passing of Hatton. As the last bell ended, the quiet gave way to a roar of "there's only one Hatton".
The competition within the ring equaled the sentiment. The eventual winner was knocked down by a left hook flush on the jaw in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage forming under his face as the pair landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of draw, 115-113 and 115-112, but both fighters were persistent and courageous, all the qualities their idol the legend stood for.
