British light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the techniques didn't complement each other, with constant grabbing marring the action.
The scoring judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my finest display but what do champions do? They win," Buatsi commented in the ring as he was greeted with boos from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Several boxing experts thought Parker, 31, should have been awarded the victory but the contender experienced the second defeat of his 28-match career.
The Champion Earns Victory in Slow Affair
His career development has stalled in recent times through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who operated with a tight guard to edge the better rounds.
The spectators' acceptance decreased amid frequent holding, with the challenger falling to the floor several times during grappling.
Parker closed well, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Arthur overcame Rea by split-decision in a outstanding all-Manchester battle to secure the continental belt.
The stadium fell silent for a extremely emotional memorial prior to the bout as ten bells rang in remembrance of local hero Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet transitioned to a roar of "only one Hatton".
The action during the bout matched the feeling. The eventual winner was sent to the canvas by a powerful punch directly to the face in the initial stage, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was marked up early, noticeable damage developing under his orbital area as the fighters landed powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the verdict with official scores of 114-114, close margin and clearer margin, but both fighters were relentless and brave, exactly what their hero Hatton embodied.
