British light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker found success, delivering effective individual punches in a match where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They win," the victor stated in the ringside as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Various analysts thought the challenger, 31, merited the decision but the Derbyshire fighter experienced the second setback of his 28-bout career.
Buatsi Secures Triumph in Uneventful Match
Buatsi's progress has stalled in recent years through management issues and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to edge the more effective rounds.
The audience's tolerance wore thin because of constant clinching, with the challenger slipping to the canvas several times during grappling.
Parker finished the stronger, but several stanzas were hard to judge. However, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Arthur beat Rea by divided decision in a outstanding local rivalry battle to win the European light heavyweight championship.
The venue fell silent for a extremely emotional tribute prior to the bout as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised boxing event in the city since Hatton's death. As the final chime ended, the silence transitioned to a roar of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. Arthur was floored by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged soon, noticeable damage appearing under his orbital area as the fighters landed punishing blows.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over 12 rounds.
The victor edged the verdict with judges scorecards of even score, close margin and wider gap, but each competitor were relentless and courageous, everything their inspiration Hatton stood for.
