UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the techniques failed to mesh, with constant grabbing spoiling the action.
The scoring judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They succeed," the victor stated in the ring as he was faced disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the decision but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Earns Victory in Uneventful Match
His career development has stalled in recent years through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to overcommit in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to barely take the cleaner stanzas.
The crowd's patience wore thin because of constant clinching, with Parker going down multiple occasions during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry battle to win the continental championship.
The venue grew hushed for a extremely emotional tribute ahead of the contest as a bell ceremony tolled in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell ended, the quiet transitioned to a shout of "there's just one Hatton".
The action inside the ropes equaled the feeling. Arthur was floored by a significant blow squarely on the chin in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured soon, heavy swelling developing under his orbital area as the duo landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur barely secured the result with judges scorecards of even score, 115-113 and clearer margin, but both fighters were determined and brave, exactly what their inspiration the legend represented.
