UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a contest where the styles failed to mesh, with frequent clinching disrupting the action.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but what do champions do? They prevail," the victor commented in the ring as he was greeted with boos from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed the challenger, 31, merited the decision but the contender endured the second setback of his 28-match career.
Buatsi Earns Victory in Uneventful Encounter
His career development has stalled in recent years through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who worked behind a solid defense to narrowly win the cleaner periods.
The crowd's patience wore thin because of constant clinching, with Parker going down several times during close exchanges.
The underdog finished the stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester fight to capture the European light heavyweight championship.
The arena fell silent for a extremely emotional memorial before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the last bell ended, the quiet transitioned to a roar of "there's only one Hatton".
The fighting within the ring matched the feeling. The eventual winner was sent to the canvas by a significant blow flush on the chin in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was injured soon, significant bruising appearing under his eye as the duo exchanged punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
The victor barely secured the verdict with judges scorecards of even score, 115-113 and 115-112, but both fighters were determined and full of heart, everything their hero the legend embodied.
