British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a fight where the techniques failed to mesh, with frequent clinching spoiling the fight.
The officials had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi commented in the ringside as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts thought Parker, 31, merited the win but the Derbyshire fighter endured the second loss of his 28-match professional journey.
Buatsi Secures Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and considerable time of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who operated with a tight guard to narrowly win the cleaner rounds.
The spectators' acceptance decreased amid constant clinching, with the challenger going down multiple occasions during clinches.
The underdog ended stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Arthur beat Rea by split-decision in a terrific all-Manchester fight to secure the European light-heavyweight title.
The venue grew hushed for a deeply poignant memorial prior to the bout as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime disappeared, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was knocked down by a left hook flush on the jaw in the initial stage, getting up unsteadily to his position on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured quickly, heavy swelling appearing under his eye as the duo traded powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the verdict with official scores of even score, 115-113 and 115-112, but the two athletes were relentless and full of heart, everything their inspiration the legend embodied.
