UK light-heavyweight Joshua Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a match where the styles failed to mesh, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi stated in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Various analysts believed the challenger, 31, deserved the win but the British boxer suffered the second defeat of his 28-match career.
The Champion Secures Win in Slow Match
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to achieve expectations - maybe in what feels like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a tight guard to edge the better periods.
The spectators' acceptance diminished because of frequent holding, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by split-decision in a terrific all-Manchester contest to win the European light-heavyweight title.
The venue grew hushed for a deeply poignant memorial before the fight as 10 bells sounded in honor of the city's native Ricky Hatton, who died 46 years old in September.
It was the premier televised fight night in the location since Hatton's passing. As the final chime ended, the stillness transitioned to a chant of "only one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was floored by a significant blow squarely on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured soon, noticeable damage forming under his orbital area as the fighters traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
The victor barely secured the decision with official scores of 114-114, 115-113 and 115-112, but both fighters were determined and brave, all the qualities their inspiration Hatton embodied.
