UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive ten-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the approaches didn't complement each other, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," Buatsi said in the ring as he was faced boos from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Various analysts believed the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second loss of his 28-fight career.
The Champion Earns Victory in Dull Match
The fighter's advancement has stalled in recent times through contract problems and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He started carefully, with neither fighter willing to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger slipping to the canvas several times during clinches.
The underdog ended stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester battle to win the European light-heavyweight title.
The arena became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the concluding sound faded, the stillness gave way to a roar of "only one Ricky Hatton".
The competition during the bout equaled the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged soon, significant bruising developing under his eye as the duo exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor edged the verdict with judges scorecards of draw, 115-113 and clearer margin, but each competitor were relentless and courageous, all the qualities their inspiration the legend stood for.
