British light heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a tight 10-round bout at the boxing arena.
But the challenger landed significant shots, landing some telling clean blows in a fight where the approaches didn't complement each other, with constant grabbing disrupting the contest.
Two judges had it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," the victor commented in the arena as he was met by boos from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Various analysts believed Parker, 31, merited the decision but the British boxer suffered the second setback of his 28-match career.
The Champion Claims Triumph in Uneventful Encounter
His career development has plateaued 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 realize his potential - maybe in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter willing to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who utilized a protective stance to edge the more effective rounds.
The audience's tolerance decreased because of regular grabbing, with Parker going down multiple occasions during grappling.
Parker closed well, but many rounds were challenging to assess. However, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a outstanding all-Manchester contest to secure the continental championship.
The venue became quiet for a profoundly moving tribute prior to the bout as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's passing. As the last bell disappeared, the stillness became a shout of "there's only one Ricky Hatton".
The action during the bout reflected the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged early, significant bruising developing under his eye as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
The victor barely secured the decision with official scores of draw, narrow lead and wider gap, but the two athletes were persistent and brave, everything their idol Hatton represented.
