British light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight ten-round fight at the Manchester venue.
But the challenger found success, connecting with powerful individual punches in a match where the techniques didn't complement each other, with frequent clinching marring the fight.
Two judges judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They prevail," the victor stated in the ringside as he was met by disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts believed the challenger, thirty-one, deserved the decision but the British boxer suffered the second setback of his 28-fight career.
Buatsi Secures Win in Slow Match
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter willing to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who operated with a solid defense to narrowly win the cleaner rounds.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Nevertheless, former world champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent all-Manchester contest to secure the continental championship.
The arena grew hushed for a deeply poignant tribute ahead of the contest as 10 bells sounded in remembrance of local hero Ricky Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the urban center since Hatton's death. As the concluding sound disappeared, the quiet gave way to a roar of "there's just one Hatton".
The action within the ring reflected the feeling. The eventual winner was floored by a significant blow squarely on the face in the second round, standing uncertainly to his position on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured quickly, noticeable damage forming under his orbital area as the pair traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
Arthur narrowly won the verdict with official scores of even score, narrow lead and 115-112, but the two athletes were relentless and brave, all the qualities their hero Hatton embodied.
