British light-heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a competitive 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a fight where the styles didn't complement each other, with frequent clinching disrupting the fight.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers believed the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-match career.
The Champion Earns Victory in Uneventful Affair
His career development has slowed in recent times through management issues and considerable time 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 ultimate opportunity - under recent support.
He began cautiously, with both competitors prepared to overcommit in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who operated with a protective stance to edge the more effective stanzas.
The spectators' acceptance decreased during regular grabbing, with Parker falling to the floor various instances during grappling.
Parker ended stronger, but many rounds were difficult to score. However, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry contest to secure the European light heavyweight belt.
The arena grew hushed for a profoundly moving tribute before the fight as ten bells tolled in memory of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the location since the passing of Hatton. As the final chime ended, the quiet gave way to a chant of "there's only one Ricky Hatton".
The competition during the bout equaled the emotion. The eventual winner was floored by a significant blow directly to the chin in the initial stage, rising shakily to his position on the referee's count.
But the traditional red attire of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was injured quickly, heavy swelling developing under his face as the fighters landed powerful shots.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
Arthur edged the verdict with official scores of even score, 115-113 and clearer margin, but the two athletes were persistent and brave, all the qualities their hero the legend represented.
