British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a match where the approaches didn't complement each other, with constant grabbing marring the contest.
Two judges had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi commented in the arena as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-fight career.
The Champion Claims Triumph in Slow Match
His career development has slowed in recent years through management issues and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a tight guard to narrowly win the better stanzas.
The spectators' acceptance wore thin amid regular grabbing, with Parker going down various instances during grappling.
Parker closed well, but many rounds were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester battle to capture the European light heavyweight belt.
The arena became quiet for a deeply poignant memorial before the fight as ten bells tolled in remembrance of local hero Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a significant blow directly to the face in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was injured early, significant bruising forming under his face as the duo traded hard punches.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over the full distance.
Arthur barely secured the result with official scores of even score, narrow lead and 115-112, but each competitor were determined and full of heart, everything their hero Hatton stood for.
