British light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a competitive ten-round fight at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches never quite clicked, with excessive holding disrupting the contest.
The officials had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what defines champions? They prevail," the victor stated in the ring as he was met by disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the win but the contender experienced the second setback of his 28-match career.
Buatsi Earns Victory in Slow Affair
Buatsi's progress has plateaued in recent years through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to edge the cleaner stanzas.
The audience's tolerance decreased during frequent holding, with Parker going down several times during clinches.
Parker closed well, but many rounds were challenging to assess. However, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding local rivalry contest to secure the continental title.
The venue grew hushed for a profoundly moving memorial prior to the bout as 10 bells rang in honor of local hero Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the location since Hatton's death. As the concluding sound faded, the silence became a chant of "there's only one Hatton".
The action inside the ropes reflected the feeling. The eventual winner was floored by a left hook flush on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored equipment of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was damaged early, significant bruising developing under his face as the pair landed hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the verdict with judges scorecards of draw, 115-113 and 115-112, but each competitor were relentless and brave, all the qualities their idol the legend represented.
