UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight ten-round contest at the boxing arena.
But the challenger found success, delivering effective clean blows in a match where the techniques didn't complement each other, with excessive holding spoiling the contest.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what do champions do? They win," the victor stated in the ring as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Several boxing experts believed the challenger, 31, merited the victory but the contender endured the second loss of his 28-bout career.
The Champion Secures Win in Uneventful Encounter
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to narrowly win the better rounds.
The spectators' acceptance wore thin amid regular grabbing, with the challenger going down several times during grappling.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Arthur overcame Rea by divided decision in a terrific local rivalry contest to win the European light heavyweight championship.
The venue fell silent for a deeply poignant memorial prior to the bout as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime faded, the stillness transitioned to a chant of "there's just one Hatton".
The competition during the bout reflected the feeling. The eventual winner was knocked down by a powerful punch directly to the face in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was damaged early, heavy swelling appearing under his eye as the duo traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor edged the verdict with scoring tallies of 114-114, close margin and 115-112, but each competitor were relentless and full of heart, all the qualities their hero Hatton stood for.
