British light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a fight where the techniques failed to mesh, with excessive holding spoiling the action.
The scoring judges scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," Buatsi stated in the arena as he was met by negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Claims Triumph in Uneventful Match
His career development has stalled in the past few years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, 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 slipping to the canvas multiple occasions during clinches.
The underdog closed well, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Inspires Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by divided decision in a excellent local rivalry contest to secure the European light heavyweight belt.
The stadium became quiet for a deeply poignant tribute before the fight as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised boxing event in the location since Hatton's death. As the final chime ended, the stillness became a chant of "only one Ricky Hatton".
The fighting during the bout matched the feeling. Arthur was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was injured early, significant bruising developing under his orbital area as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur edged the result with official scores of 114-114, 115-113 and 115-112, but each competitor were persistent and courageous, everything their idol the legend represented.
