British light heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger found success, delivering effective single shots in a match where the approaches failed to mesh, with excessive holding disrupting the action.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi commented in the ringside as he was faced negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the win but the British boxer suffered the second defeat of his 28-match career.
Buatsi Claims Victory in Slow Match
The fighter's advancement has slowed in recent years through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, 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 tight guard to barely take the better periods.
The audience's tolerance wore thin because of frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to win the European light heavyweight championship.
The venue became quiet for a profoundly moving homage before the fight as a bell ceremony tolled in memory of Manchester's own Hatton, who died at age 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime faded, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The action within the ring equaled the feeling. The eventual winner was sent to the canvas by a left hook squarely on the face in the second round, standing uncertainly to his position on the eight count.
But the red-colored attire of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was damaged quickly, noticeable damage forming under his orbital area as the pair exchanged powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the decision with official scores of draw, close margin and 115-112, but each competitor were relentless and brave, all the qualities their inspiration the legend represented.
