UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close ten-round contest at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a fight where the techniques failed to mesh, with frequent clinching spoiling the fight.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi stated in the ring as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th pro success.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the contender experienced the second defeat of his 28-fight professional journey.
Buatsi Claims Victory in Slow Match
His career development has slowed in recent years through management issues and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second fight at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a protective stance to edge the more effective periods.
The audience's tolerance diminished during frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog closed well, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight championship.
The arena grew hushed for a deeply poignant tribute ahead of the contest as ten bells tolled in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound faded, the quiet became a shout of "only one Hatton".
The competition inside the ropes equaled the feeling. Arthur was floored by a left hook squarely on the jaw in the initial stage, rising shakily to his footing on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured quickly, heavy swelling developing under his face as the fighters landed punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and full of heart, all the qualities their hero the legend embodied.
