UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a match where the styles failed to mesh, with frequent clinching disrupting the action.
The scoring judges judged it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best display but what defines champions? They win," Buatsi said in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought the challenger, thirty-one, deserved the decision but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
The Champion Earns Win in Slow Match
His career development has slowed in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who utilized a solid defense to edge the cleaner periods.
The spectators' acceptance decreased amid constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent local rivalry contest to secure the European light heavyweight belt.
The stadium fell silent for a deeply poignant tribute ahead of the contest as a bell ceremony tolled in memory of local hero Hatton, who passed away aged 46 in last month.
It was the premier televised boxing event in the location since Hatton's death. As the final chime faded, the silence gave way to a shout of "there's just one Hatton".
The competition during the bout matched the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was damaged soon, heavy swelling appearing under his eye as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the decision with scoring tallies of even score, narrow lead and clearer margin, but both fighters were determined and brave, all the qualities their idol Hatton represented.
