UK light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the styles never quite clicked, with excessive holding disrupting the contest.
Two judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They prevail," Buatsi commented in the arena as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th pro success.
Various analysts thought the challenger, 31, merited the victory but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Claims Win in Uneventful Affair
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with the two boxers ready to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a tight guard to edge the more effective rounds.
The audience's tolerance wore thin because of frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Homage Fuels All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to secure the continental championship.
The arena grew hushed for a extremely emotional tribute prior to the bout as a bell ceremony rang in honor of local hero Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the city since Hatton's passing. As the final chime disappeared, the silence transitioned to a shout of "there's only one Ricky Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a left hook directly to the face in the early round, getting up unsteadily to his footing on the referee's count.
But the traditional red attire of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was injured early, heavy swelling appearing under his face as the fighters traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur barely secured the verdict with scoring tallies of 114-114, close margin and clearer margin, but the two athletes were persistent and full of heart, exactly what their idol Hatton represented.
