British light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the styles didn't complement each other, with constant grabbing spoiling the contest.
The officials scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best performance but what defines champions? They succeed," the victor said in the arena as he was greeted with negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, merited the decision but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
The Champion Earns Win in Slow Affair
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to barely take the cleaner periods.
The crowd's patience wore thin during regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry battle to secure the European light-heavyweight title.
The stadium became quiet for a deeply poignant tribute before the fight as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the urban center since Hatton's death. As the final chime faded, the silence became a shout of "there's just one Hatton".
The competition during the bout equaled the emotion. The eventual winner was floored by a left hook directly to the face in the initial stage, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up early, significant bruising forming under his orbital area as the pair traded punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over the full distance.
Arthur narrowly won the decision with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were determined and courageous, all the qualities their inspiration the legend embodied.
