British light-heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round bout at the Manchester venue.
But Parker found success, delivering effective clean blows in a match where the styles didn't complement each other, with excessive holding spoiling the contest.
The officials judged it 96-94 for to the winner, with the third a even score.
"It wasn't my finest performance but what defines champions? They win," Buatsi said in the ring as he was met by negative reactions from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, deserved the decision but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
Buatsi Earns Victory in Uneventful Encounter
Buatsi's progress has plateaued in recent years through management issues and long spells of inactivity.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter willing to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to barely take the better stanzas.
The crowd's patience wore thin because of frequent holding, with Parker going down multiple occasions during clinches.
Parker closed well, but many rounds were difficult to score. However, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to secure the continental belt.
The venue fell silent for a extremely emotional tribute before the fight as 10 bells tolled in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the premier televised match card in the urban center since the passing of Hatton. As the final chime faded, the quiet became a shout of "there's only one Hatton".
The competition during the bout matched the feeling. Arthur was knocked down by a left hook squarely on the chin in the second round, rising shakily to his footing on the count of eight.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured early, significant bruising developing under his orbital area as the duo landed punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace seldom decreased over 12 rounds.
Arthur barely secured the verdict with official scores of draw, narrow lead and clearer margin, but each competitor were determined and courageous, everything their inspiration the legend represented.
