British light heavyweight Joshua Buatsi earned a win yet underwhelming debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a match where the approaches didn't complement each other, with constant grabbing disrupting the contest.
The scoring judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my best display but what defines champions? They prevail," the victor commented in the ringside as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Various analysts thought Parker, thirty-one, merited the win but the contender experienced the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who utilized a protective stance to barely take the better periods.
The crowd's patience diminished because of frequent holding, with the challenger slipping to the canvas various instances during clinches.
Parker ended stronger, but many rounds were difficult to score. However, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Arthur beat Rea by divided decision in a outstanding local rivalry fight to win the European light heavyweight belt.
The venue grew hushed for a profoundly moving homage prior to the bout as ten bells sounded in memory of the city's native Hatton, who died 46 years old in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound faded, the quiet gave way to a roar of "only one Hatton".
The fighting during the bout reflected the feeling. The eventual winner was sent to the canvas by a significant blow flush on the chin in the early round, rising shakily to his feet on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, significant bruising appearing under his eye as the pair traded powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor barely secured the decision with judges scorecards of even score, 115-113 and wider gap, but each competitor were determined and courageous, everything their idol Hatton embodied.
