UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the techniques didn't complement each other, with frequent clinching disrupting the contest.
The scoring judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my best showing but what do champions do? They prevail," the victor said in the ringside as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Various analysts felt the challenger, 31, deserved the win but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Claims Win in Slow Match
His career development has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who operated with a protective stance to edge the better periods.
The audience's tolerance diminished during regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just one round.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to secure the continental belt.
The venue became quiet for a deeply poignant homage prior to the bout as ten bells sounded in memory of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's death. As the concluding sound faded, the silence became a roar of "only one Hatton".
The fighting within the ring equaled the feeling. Arthur was floored by a significant blow directly to the jaw in the initial stage, rising shakily to his feet on the count of eight.
But the traditional red attire of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was marked up early, heavy swelling appearing under his face as the pair traded powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor edged the verdict with official scores of even score, 115-113 and 115-112, but both fighters were persistent and brave, all the qualities their inspiration Hatton embodied.
