UK light heavyweight Joshua Buatsi made a winning yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a competitive ten-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the approaches failed to mesh, with constant grabbing marring the contest.
The scoring judges scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor stated in the ring as he was met by boos from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts believed Parker, 31, deserved the victory but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Claims Win in Uneventful Match
Buatsi's progress has slowed in the past few years through contract problems and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with both competitors prepared to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who worked behind a tight guard to barely take the better periods.
The crowd's patience wore thin because of constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a terrific all-Manchester contest to secure the European light-heavyweight title.
The arena grew hushed for a deeply poignant homage before the fight as ten bells rang in memory of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the concluding sound ended, the quiet transitioned to a chant of "there's only one Hatton".
The competition within the ring matched the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling developing under his face as the duo exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the decision with scoring tallies of draw, close margin and clearer margin, but each competitor were determined and courageous, everything their hero Hatton embodied.
