British light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a competitive 10-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They win," the victor stated in the ringside as he was faced negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Several boxing experts thought Parker, thirty-one, should have been awarded the decision but the British boxer endured the second defeat of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Encounter
The fighter's advancement has plateaued in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under recent support.
He started carefully, with neither fighter willing to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who utilized a solid defense to edge the cleaner rounds.
The spectators' acceptance decreased amid regular grabbing, with Parker going down multiple occasions during grappling.
Parker closed well, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent local rivalry contest to win the continental title.
The venue grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who died aged 46 in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the final chime faded, the silence gave way to a chant of "there's only one Hatton".
The action during the bout reflected the emotion. The eventual winner was floored by a significant blow squarely on the face in the initial stage, rising shakily to his position on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured early, heavy swelling appearing under his face as the duo landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the verdict with official scores of even score, close margin and 115-112, but each competitor were determined and courageous, exactly what their hero the legend embodied.
