British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a tight 10-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a match where the techniques never quite clicked, with frequent clinching marring the action.
The officials had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They prevail," the victor said in the ringside as he was met by disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Several boxing experts believed Parker, 31, deserved the victory but the contender endured the second defeat of his 28-match career.
Buatsi Earns Triumph in Uneventful Encounter
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - possibly in what feels like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers ready to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who worked behind a protective stance to barely take the better stanzas.
The spectators' acceptance decreased amid constant clinching, with Parker falling to the floor various instances during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester contest to secure the continental title.
The arena became quiet for a extremely emotional tribute prior to the bout as ten bells sounded in remembrance of the city's native Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the city since Hatton's passing. As the last bell faded, the stillness became a shout of "there's only one Ricky Hatton".
The action during the bout reflected the feeling. Arthur was knocked down by a significant blow directly to the jaw in the second round, rising shakily to his feet on the referee's count.
But the red-colored attire of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was damaged soon, heavy swelling appearing under his eye as the pair landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur barely secured the verdict with official scores of draw, close margin and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero the legend represented.
