UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight 10-round contest at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a fight where the techniques didn't complement each other, with frequent clinching marring the contest.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest showing but how do champions respond? They win," Buatsi stated in the arena as he was met by disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Earns Win in Dull Match
His career development has plateaued in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors prepared to take risks in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who utilized a protective stance to barely take the more effective stanzas.
The crowd's patience diminished during frequent holding, with Parker going down various instances during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the continental championship.
The stadium 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 at age 46 in recently.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound ended, the quiet became a roar of "there's just one Hatton".
The fighting within the ring equaled the emotion. The eventual winner was sent to the canvas by a left hook directly to the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged soon, noticeable damage developing under his face as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur edged the verdict with official scores of even score, narrow lead and clearer margin, but both fighters were persistent and courageous, all the qualities their hero the legend embodied.
