UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But Parker found success, landing some telling single shots in a contest where the styles never quite clicked, with excessive holding marring the contest.
Two judges had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They win," the victor said in the arena as he was faced negative reactions from the crowd.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, merited the decision but the Derbyshire fighter suffered the second loss of his 28-bout professional journey.
Buatsi Earns Victory in Slow Match
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who utilized a tight guard to narrowly win the better periods.
The crowd's patience wore thin because of constant clinching, with Parker going down various instances during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. Although, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Arthur beat Rea by divided decision in a terrific Manchester vs Manchester battle to secure the European light heavyweight title.
The stadium grew hushed for a profoundly moving memorial ahead of the contest as 10 bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a roar of "there's just one Hatton".
The action inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red shorts of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was injured early, significant bruising forming under his eye as the fighters exchanged punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur barely secured the decision with official scores of draw, 115-113 and wider gap, but each competitor were determined and courageous, exactly what their hero Hatton stood for.
