UK light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a match where the techniques failed to mesh, with constant grabbing disrupting the contest.
Two judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best display but how do champions respond? They succeed," Buatsi stated in the arena as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought Parker, 31, merited the win but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Claims Victory in Uneventful Encounter
His career development has slowed in recent times through management issues and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who worked behind a tight guard to barely take the more effective rounds.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor several times during grappling.
Parker closed well, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by close verdict in a outstanding local rivalry battle to secure the continental title.
The arena fell silent for a profoundly moving homage before the fight as 10 bells sounded in remembrance of Manchester's own Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a shout of "there's just one Ricky Hatton".
The action within the ring matched the feeling. Arthur was sent to the canvas by a left hook directly to the face in the second round, rising shakily to his footing on the eight count.
But the traditional red attire of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising developing under his eye as the pair traded hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of even score, narrow lead and wider gap, but both fighters were determined and full of heart, exactly what their idol the legend embodied.
