UK light-heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close 10-round contest at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a contest where the styles never quite clicked, with frequent clinching spoiling the contest.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," Buatsi stated in the ringside as he was faced disapproval from the crowd.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers thought Parker, 31, deserved the win but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Earns Victory in Slow Encounter
The fighter's advancement has stalled in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to narrowly win the better periods.
The audience's tolerance decreased during frequent holding, with Parker falling to the floor various instances during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Arthur beat Bradley Rea by split-decision in a excellent local rivalry battle to win the continental championship.
The venue fell silent for a extremely emotional memorial before the fight as ten bells tolled in honor of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the quiet transitioned to a shout of "only one Hatton".
The action during the bout reflected the sentiment. The eventual winner was knocked down by a significant blow directly to the jaw in the second round, standing uncertainly to his feet on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, heavy swelling forming under his face as the duo landed punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over the full distance.
The victor edged the result with judges scorecards of even score, narrow lead and clearer margin, but each competitor were persistent and courageous, all the qualities their idol Hatton represented.
