UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the techniques didn't complement each other, with frequent clinching disrupting the fight.
The officials had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, deserved the victory but the British boxer endured the second setback of his 28-match professional journey.
The Champion Claims Win in Uneventful Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the better rounds.
The crowd's patience wore thin amid constant clinching, with Parker going down several times during clinches.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a excellent local rivalry contest to capture the European light heavyweight belt.
The arena grew hushed for a profoundly moving tribute ahead of the contest as ten bells sounded in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound ended, the quiet gave way to a shout of "there's only one Hatton".
The fighting during the bout equaled the feeling. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, standing uncertainly to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up early, heavy swelling forming under his face as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace seldom decreased over the full distance.
The victor barely secured the decision with official scores of 114-114, 115-113 and 115-112, but both fighters were relentless and brave, all the qualities their inspiration the legend represented.
