UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a close 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with frequent clinching spoiling the contest.
Two judges had it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They succeed," the victor stated in the arena as he was met by disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts thought Parker, 31, merited the victory but the British boxer suffered the second defeat of his 28-match career.
Buatsi Claims Victory in Slow Match
His career development has slowed in recent times through contract problems and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who worked behind a tight guard to barely take the cleaner stanzas.
The spectators' acceptance diminished because of constant clinching, with Parker slipping to the canvas various instances during clinches.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the co-main event, Arthur overcame Bradley Rea by close verdict in a outstanding local rivalry battle to secure the European light-heavyweight title.
The arena became quiet for a extremely emotional homage prior to the bout as 10 bells rang in remembrance of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the city since Hatton's passing. As the final chime ended, the silence transitioned to a shout of "there's only one Ricky Hatton".
The action inside the ropes matched the sentiment. Arthur was floored by a significant blow directly to the chin in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up quickly, significant bruising forming under his orbital area as the pair traded hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the verdict with scoring tallies of 114-114, narrow lead and clearer margin, but each competitor were persistent and courageous, all the qualities their idol Hatton stood for.
