British light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a match where the techniques never quite clicked, with constant grabbing disrupting the fight.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi stated in the ring as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought Parker, 31, deserved the victory but the British boxer suffered the second defeat of his 28-bout professional journey.
Buatsi Claims Triumph in Uneventful Affair
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers ready to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who worked behind a protective stance to barely take the better rounds.
The crowd's patience wore thin because of regular grabbing, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to secure the continental title.
The arena became quiet for a deeply poignant homage prior to the bout as 10 bells tolled in honor of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The action within the ring matched the emotion. The eventual winner was floored by a left hook directly to the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red shorts of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was injured early, noticeable damage appearing under his face as the fighters exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
Arthur edged the result with official scores of even score, narrow lead and wider gap, but both fighters were relentless and full of heart, exactly what their hero Hatton represented.
