UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the approaches never quite clicked, with excessive holding marring the contest.
The officials scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest showing but what do champions do? They win," Buatsi commented in the arena as he was met by boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the win but the contender suffered the second loss of his 28-bout professional journey.
Buatsi Claims Win in Uneventful Match
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under fresh management.
He started carefully, with neither fighter willing to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to narrowly win the better stanzas.
The audience's tolerance wore thin amid frequent holding, with Parker going down various instances during grappling.
The underdog closed well, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Motivates All-Manchester Contest
In the co-main event, Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester battle to capture the continental title.
The stadium grew hushed for a deeply poignant tribute before the fight as 10 bells rang in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the final chime ended, the quiet gave way to a chant of "only one Ricky Hatton".
The action inside the ropes reflected the feeling. The eventual winner was floored by a significant blow flush on the chin in the early round, standing uncertainly to his feet on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was damaged soon, significant bruising forming under his face as the pair landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
The victor edged the verdict with official scores of draw, narrow lead and wider gap, but each competitor were persistent and full of heart, all the qualities their idol Hatton embodied.
