British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the techniques failed to mesh, with frequent clinching spoiling the fight.
Two judges judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best display but what do champions do? They prevail," the victor stated in the ringside as he was greeted with boos from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the victory but the British boxer suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Encounter
Buatsi's progress has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who utilized a solid defense to narrowly win the cleaner stanzas.
The audience's tolerance decreased during regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but multiple periods were hard to judge. However, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific all-Manchester fight to capture the European light heavyweight championship.
The venue grew hushed for a extremely emotional tribute prior to the bout as a bell ceremony sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the final chime disappeared, the silence transitioned to a roar of "only one Ricky Hatton".
The action during the bout reflected the emotion. Arthur was knocked down by a left hook squarely on the face in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, heavy swelling appearing under his orbital area as the duo exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
The victor edged the decision with scoring tallies of even score, narrow lead and clearer margin, but each competitor were determined and courageous, everything their idol Hatton represented.
