UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a close ten-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a match where the approaches never quite clicked, with excessive holding marring the action.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," the victor stated in the ringside as he was met by boos from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, should have been awarded the victory but the contender experienced the second loss of his 28-match professional journey.
Buatsi Claims Triumph in Slow Match
The fighter's advancement has stalled in the past few years through contract problems and long spells of inactivity.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He began cautiously, with both competitors willing to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who operated with a protective stance to edge the better stanzas.
The audience's tolerance diminished amid constant clinching, with Parker falling to the floor several times during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. Although, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Contest
In the supporting bout, Arthur overcame Bradley Rea by close verdict in a excellent local rivalry contest to secure the European light-heavyweight title.
The arena fell silent for a extremely emotional homage before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who died 46 years old in September.
It was the premier televised boxing event in the city since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a roar of "there's just one Hatton".
The competition inside the ropes reflected the emotion. Arthur was knocked down by a left hook directly to the chin in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged early, significant bruising developing under his face as the pair traded punishing blows.
The challenger, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor narrowly won the result with scoring tallies of draw, 115-113 and wider gap, but both fighters were relentless and brave, exactly what their inspiration the legend represented.
