UK light heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close 10-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques failed to mesh, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," the victor stated in the ringside as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Various analysts felt Parker, thirty-one, deserved the decision but the contender endured the second loss of his 28-bout professional journey.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has plateaued in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter willing to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a tight guard to narrowly win the cleaner stanzas.
The spectators' acceptance decreased because of frequent holding, with Parker falling to the floor several times during grappling.
Parker closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Motivates Local Rivalry Fight
In the co-main event, Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to win the continental championship.
The stadium grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the location since Hatton's passing. As the last bell faded, the silence transitioned to a shout of "there's only one Hatton".
The fighting within the ring matched the sentiment. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the second round, rising shakily to his footing on the count of eight.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising developing under his face as the duo exchanged punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
The victor narrowly won the decision with scoring tallies of 114-114, close margin and 115-112, but both fighters were relentless and full of heart, all the qualities their hero Hatton stood for.
