UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close ten-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a match where the approaches never quite clicked, with excessive holding spoiling the action.
Two judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi commented in the ring as he was met by negative reactions from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers believed Parker, 31, merited the decision but the contender endured the second loss of his 28-match professional journey.
The Champion Secures Victory in Dull Encounter
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter ready to overcommit in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to narrowly win the cleaner rounds.
The crowd's patience diminished because of frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog closed well, but multiple periods were difficult to score. However, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to secure the continental championship.
The venue fell silent for a deeply poignant memorial ahead of the contest as ten bells sounded in remembrance of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised fight night in the location since Hatton's passing. As the concluding sound ended, the silence became a roar of "there's only one Hatton".
The action during the bout reflected the feeling. The eventual winner was floored by a significant blow squarely on the chin in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, significant bruising developing under his face as the duo landed powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor barely secured the result with scoring tallies of draw, close margin and clearer margin, but each competitor were determined and brave, exactly what their hero Hatton stood for.
