British light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a close 10-round fight at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a match where the approaches never quite clicked, with excessive holding marring the action.
Two judges judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," Buatsi said in the ring as he was faced negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Various analysts thought Parker, 31, merited the victory but the contender experienced the second setback of his 28-match professional journey.
Buatsi Secures Triumph in Dull Affair
The fighter's advancement has plateaued in recent years through management issues and long spells of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who operated with a solid defense to narrowly win the better stanzas.
The audience's tolerance decreased amid regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David 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 co-main event, Lyndon Arthur defeated Rea by split-decision in a terrific all-Manchester battle to capture the European light heavyweight title.
The venue became quiet for a extremely emotional tribute before the fight as 10 bells tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime disappeared, the stillness gave way to a chant of "only one Ricky Hatton".
The competition during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured soon, noticeable damage forming under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor barely secured the verdict with judges scorecards of even score, narrow lead and wider gap, but both fighters were persistent and courageous, everything their hero Hatton represented.
