UK light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a contest where the approaches didn't complement each other, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but what do champions do? They win," Buatsi stated in the arena as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, deserved the win but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Claims Victory in Dull Match
Buatsi's progress has plateaued in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to realize his potential - maybe in what feels like a last chance - under fresh management.
He started carefully, with neither fighter willing to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the more effective rounds.
The crowd's patience decreased amid frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry fight to secure the European light heavyweight championship.
The stadium fell silent for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell faded, the quiet became a roar of "only one Ricky Hatton".
The fighting during the bout reflected the emotion. The eventual winner was knocked down by a powerful punch directly to the chin in the early round, getting up unsteadily to his footing on the referee's count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured early, heavy swelling forming under his eye as the fighters exchanged punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and full of heart, exactly what their inspiration the legend represented.
