British light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a match where the approaches never quite clicked, with constant grabbing spoiling the action.
Two judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," the victor stated in the arena as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the win but the Derbyshire fighter endured the second defeat of his 28-bout career.
The Champion Claims Victory in Uneventful Encounter
The fighter's advancement has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers willing to take risks in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to edge the cleaner stanzas.
The spectators' acceptance wore thin because of frequent holding, with the challenger falling to the floor several times during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Memorial Fuels Manchester Derby Battle
In the co-main event, Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester fight to win the continental belt.
The arena fell silent for a profoundly moving tribute ahead of the contest as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime faded, the silence became a roar of "only one Hatton".
The competition inside the ropes reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was injured soon, heavy swelling forming under his eye as the fighters traded powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur narrowly won the result with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and courageous, all the qualities their idol the legend embodied.
