UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a close ten-round bout at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches didn't complement each other, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They succeed," Buatsi stated in the ring as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, merited the decision but the British boxer experienced the second setback of his 28-bout career.
Buatsi Claims Triumph in Dull Match
His career development has slowed in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who operated with a tight guard to edge the more effective periods.
The spectators' acceptance decreased because of frequent holding, with Parker falling to the floor several times during grappling.
Parker closed well, but several stanzas were hard to judge. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the European light heavyweight belt.
The venue grew hushed for a profoundly moving tribute prior to the bout as 10 bells sounded in memory of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the city since Hatton's passing. As the final chime faded, the quiet gave way to a roar of "there's just one Hatton".
The action during the bout equaled the emotion. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was damaged quickly, noticeable damage forming under his face as the pair traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the result with scoring tallies of draw, close margin and clearer margin, but the two athletes were persistent and brave, exactly what their hero the legend stood for.
