UK light-heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a match where the styles failed to mesh, with frequent clinching spoiling the contest.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi commented in the arena as he was faced boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the decision but the British boxer suffered the second defeat of his 28-bout professional journey.
The Champion Claims Win in Slow Affair
His career development has plateaued in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors ready to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner stanzas.
The audience's tolerance decreased during frequent holding, with Parker going down various instances during grappling.
The underdog finished the stronger, but several stanzas were hard to judge. Although, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor advances, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the co-main event, Arthur beat Bradley Rea by close verdict in a terrific local rivalry battle to win the continental title.
The stadium fell silent for a profoundly moving homage before the fight as 10 bells rang in memory of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the last bell ended, the stillness transitioned to a roar of "only one Hatton".
The action during the bout matched the emotion. Arthur was sent to the canvas by a left hook squarely on the face in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured early, significant bruising appearing under his face as the pair traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor narrowly won the result with official scores of even score, narrow lead and wider gap, but each competitor were persistent and brave, all the qualities their inspiration the legend stood for.
