British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the techniques failed to mesh, with frequent clinching spoiling the contest.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They win," the victor stated in the ring as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the victory but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Secures Victory in Dull Encounter
His career development has slowed in the past few years through promotional disputes and considerable time of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who worked behind a tight guard to barely take the more effective rounds.
The spectators' acceptance decreased during regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but many rounds were difficult to score. However, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester fight to capture the continental championship.
The arena fell silent for a profoundly moving memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Hatton, who passed away 46 years old in last month.
It was the premier televised fight night in the city since Hatton's passing. As the concluding sound disappeared, the stillness became a chant of "there's just one Hatton".
The competition during the bout reflected the feeling. Arthur was sent to the canvas by a left hook directly to the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured quickly, heavy swelling forming under his face as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the verdict with official scores of 114-114, narrow lead and wider gap, but each competitor were determined and brave, exactly what their inspiration Hatton embodied.
