British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive ten-round bout at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a contest where the techniques didn't complement each other, with constant grabbing spoiling the action.
Two judges scored it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," the victor said in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Earns Victory in Uneventful Affair
Buatsi's progress has stalled in recent times through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to overcommit in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who operated with a protective stance to edge the more effective stanzas.
The spectators' acceptance wore thin during frequent holding, with Parker going down various instances during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Arthur beat Bradley Rea by divided decision in a terrific local rivalry fight to capture the European light heavyweight championship.
The arena grew hushed for a deeply poignant tribute ahead of the contest as a bell ceremony rang in memory of the city's native Hatton, who succumbed at age 46 in last month.
It was the premier televised match card in the location since Hatton's death. As the last bell ended, the silence gave way to a chant of "only one Hatton".
The action within the ring reflected the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, getting up unsteadily to his footing on the referee's count.
But the traditional red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was damaged quickly, heavy swelling developing under his face as the duo exchanged hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the result with scoring tallies of draw, narrow lead and clearer margin, but each competitor were relentless and courageous, exactly what their inspiration Hatton embodied.
