UK light heavyweight Joshua Buatsi earned a win yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close ten-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the styles didn't complement each other, with excessive holding disrupting the contest.
The officials had it 96-94 to the winner, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the ringside as he was met by disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts felt Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
The Champion Earns Win in Slow Match
The fighter's advancement has slowed in the past few years through contract problems and long spells 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 feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to barely take the cleaner periods.
The audience's tolerance decreased during regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester battle to win the continental belt.
The venue became quiet for a extremely emotional homage ahead of the contest as a bell ceremony rang in honor of local hero Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the last bell faded, the quiet transitioned to a roar of "only one Ricky Hatton".
The fighting during the bout equaled the emotion. Arthur was sent to the canvas by a powerful punch directly to the face in the initial stage, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up quickly, significant bruising forming under his eye as the duo traded powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over the full distance.
The victor edged the result with judges scorecards of even score, 115-113 and 115-112, but both fighters were persistent and brave, exactly what their inspiration Hatton stood for.
