British light heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round contest at the Manchester venue.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the styles failed to mesh, with constant grabbing disrupting the fight.
The officials scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest performance but how do champions respond? They win," the victor said in the ring as he was faced disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought the challenger, 31, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-match career.
Buatsi Earns Win in Slow Affair
His career development has slowed in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under recent support.
He started carefully, with both competitors prepared to overcommit 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 during the bout, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to narrowly win the more effective periods.
The crowd's patience decreased amid regular grabbing, with Parker slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a excellent all-Manchester contest to win the European light-heavyweight belt.
The venue became quiet for a profoundly moving tribute prior to the bout as 10 bells rang in honor of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell ended, the silence gave way to a shout of "there's just one Hatton".
The competition during the bout equaled the feeling. Arthur was sent to the canvas by a significant blow squarely on the face in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, heavy swelling forming under his eye as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were determined and full of heart, everything their hero the legend embodied.
