British light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight 10-round bout at the boxing arena.
But Parker found success, connecting with powerful single shots in a match where the styles didn't complement each other, with frequent clinching disrupting the action.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best display but what do champions do? They succeed," Buatsi stated in the ring as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers believed the challenger, thirty-one, merited the decision but the British boxer suffered the second defeat of his 28-match career.
Buatsi Earns Victory in Slow Match
The fighter's advancement has slowed in recent years through management issues and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what appears to be a last chance - under fresh management.
He began cautiously, with both competitors prepared to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to narrowly win the more effective periods.
The spectators' acceptance decreased amid regular grabbing, with Parker slipping to the canvas several times during grappling.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester battle to win the European light heavyweight championship.
The venue grew hushed for a deeply poignant memorial before the fight as 10 bells rang in remembrance of local hero Ricky Hatton, who died 46 years old in last month.
It was the first major televised fight night in the city since Hatton's passing. As the final chime ended, the quiet gave way to a shout of "there's just one Ricky Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the early round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was marked up early, heavy swelling developing under his eye as the pair landed powerful shots.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the verdict with official scores of even score, 115-113 and clearer margin, but both fighters were persistent and courageous, exactly what their hero Hatton embodied.
