UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight 10-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a fight where the styles didn't complement each other, with constant grabbing spoiling the fight.
The scoring judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They succeed," the victor said in the ring as he was faced boos from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Some ringside observers felt Parker, 31, merited the decision but the British boxer endured the second loss of his 28-bout career.
Buatsi Claims Triumph in Slow Match
His career development has stalled in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who utilized a solid defense to barely take the cleaner periods.
The crowd's patience wore thin because of regular grabbing, with Parker going down various instances during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the co-main event, Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to capture the European light heavyweight title.
The stadium fell silent for a deeply poignant tribute prior to the bout as 10 bells rang in remembrance of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime disappeared, the silence became a shout of "only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was floored by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was damaged soon, significant bruising forming under his orbital area as the fighters traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur narrowly won the verdict with judges scorecards of draw, close margin and wider gap, but each competitor were persistent and full of heart, everything their hero Hatton embodied.
