UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a fight where the styles didn't complement each other, with constant grabbing spoiling the action.
The officials scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my finest display but what defines champions? They win," Buatsi said in the arena as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Various analysts thought Parker, 31, merited the decision but the British boxer experienced the second setback of his 28-bout career.
Buatsi Secures Victory in Slow Affair
The fighter's advancement has stalled in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter prepared to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to barely take the better periods.
The audience's tolerance wore thin amid frequent holding, with the challenger going down several times during clinches.
Parker finished the stronger, but several stanzas were difficult to score. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Arthur beat Bradley Rea by split-decision in a excellent all-Manchester fight to capture the European light-heavyweight title.
The venue fell silent for a extremely emotional memorial before the fight as a bell ceremony tolled in remembrance of local hero Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the city since Hatton's death. As the concluding sound faded, the silence became a shout of "there's only one Ricky Hatton".
The competition within the ring reflected the feeling. Arthur was floored by a left hook squarely on the face in the initial stage, standing uncertainly to his position on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was marked up quickly, heavy swelling appearing under his orbital area as the pair traded powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur edged the decision with scoring tallies of 114-114, close margin and 115-112, but the two athletes were relentless and brave, everything their inspiration the legend embodied.
