UK light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a contest where the approaches failed to mesh, with frequent clinching disrupting the action.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They win," Buatsi said in the ringside as he was greeted with disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts felt the challenger, 31, deserved the victory but the British boxer endured the second defeat of his 28-fight professional journey.
Buatsi Earns Win in Dull Affair
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a last chance - under recent support.
He began cautiously, with neither fighter prepared to take risks in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a solid defense to narrowly win the cleaner periods.
The spectators' acceptance decreased amid frequent holding, with the challenger going down multiple occasions during grappling.
Parker closed well, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Arthur overcame Rea by divided decision in a excellent all-Manchester contest to win the continental title.
The stadium grew hushed for a deeply poignant homage before the fight as a bell ceremony sounded in remembrance of local hero Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the last bell disappeared, the silence transitioned to a chant of "there's only one Hatton".
The action inside the ropes matched the emotion. The eventual winner was knocked down by a powerful punch squarely on the face in the early round, rising shakily to his feet on the referee's count.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured soon, significant bruising developing under his face as the fighters traded powerful shots.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor narrowly won the decision with scoring tallies of draw, narrow lead and 115-112, but the two athletes were determined and brave, all the qualities their inspiration the legend embodied.
