UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi commented in the ring as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Various analysts felt the challenger, thirty-one, merited the decision but the contender endured the second loss of his 28-match career.
The Champion Claims Win in Uneventful Affair
The fighter's advancement has plateaued in recent times through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers ready to take risks in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to barely take the more effective stanzas.
The audience's tolerance diminished during frequent holding, with the challenger falling to the floor several times during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester contest to capture the European light-heavyweight championship.
The arena grew hushed for a deeply poignant homage prior to the bout as 10 bells sounded in remembrance of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the location since Hatton's death. As the last bell disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was knocked down by a significant blow squarely on the face in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up quickly, heavy swelling developing under his face as the duo traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the verdict with scoring tallies of even score, narrow lead and 115-112, but each competitor were relentless and courageous, exactly what their inspiration Hatton represented.
