British light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive 10-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches never quite clicked, with constant grabbing disrupting the fight.
The officials judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi stated in the ringside as he was met by disapproval from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the decision but the contender endured the second setback of his 28-fight career.
The Champion Earns Win in Dull Affair
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who worked behind a tight guard to edge the cleaner periods.
The crowd's patience diminished during constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Arthur defeated Rea by close verdict in a outstanding all-Manchester contest to win the European light heavyweight championship.
The venue became quiet for a deeply poignant tribute prior to the bout as ten bells tolled in memory of the city's native Ricky Hatton, who passed away 46 years old in September.
It was the first major televised boxing event in the urban center since Hatton's death. As the final chime faded, the stillness transitioned to a chant of "there's just one Hatton".
The fighting within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the face in the early round, rising shakily to his footing on the count of eight.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was damaged quickly, noticeable damage developing under his eye as the pair traded punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over the full distance.
The victor edged the decision with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were determined and brave, everything their inspiration Hatton represented.
