UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a match where the styles didn't complement each other, with excessive holding spoiling the contest.
Two judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," Buatsi commented in the arena as he was faced boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Various analysts believed the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second loss of his 28-bout career.
Buatsi Earns Win in Dull Affair
Buatsi's progress has slowed in the past few years through contract problems and long spells of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the better rounds.
The audience's tolerance diminished amid frequent holding, with Parker falling to the floor multiple occasions during grappling.
Parker finished the stronger, but many rounds were hard to judge. Although, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a excellent local rivalry battle to secure the continental title.
The stadium became quiet for a profoundly moving tribute before the fight as 10 bells rang in memory of local hero Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the last bell faded, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a significant blow flush on the face in the early round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage appearing under his face as the fighters landed powerful shots.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the decision with judges scorecards of 114-114, narrow lead and 115-112, but each competitor were persistent and full of heart, exactly what their idol the legend stood for.
