UK light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a match where the styles never quite clicked, with constant grabbing spoiling the contest.
The officials judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best display but what defines champions? They win," Buatsi stated in the ring as he was faced negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers thought Parker, 31, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
The Champion Earns Win in Uneventful Match
Buatsi's progress has slowed in recent years through contract problems and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to narrowly win the better stanzas.
The spectators' acceptance decreased during frequent holding, with the challenger going down several times during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Homage Motivates Manchester Derby Battle
In the supporting bout, Arthur beat Rea by divided decision in a outstanding all-Manchester contest to capture the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional homage prior to the bout as ten bells sounded in memory of local hero Hatton, who passed away 46 years old in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the final chime faded, the stillness gave way to a chant of "there's just one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was knocked down by a significant blow directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was injured soon, heavy swelling appearing under his face as the fighters landed punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor barely secured the result with scoring tallies of 114-114, close margin and wider gap, but each competitor were persistent and full of heart, everything their hero the legend represented.
