UK light-heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a match where the techniques didn't complement each other, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best showing but what defines champions? They win," the victor commented in the ring as he was faced boos from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, 31, merited the victory but the Derbyshire fighter endured the second loss of his 28-fight career.
The Champion Earns Triumph in Dull Match
His career development has slowed in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who worked behind a protective stance to barely take the better rounds.
The crowd's patience decreased because of constant clinching, with Parker going down multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight belt.
The venue grew hushed for a deeply poignant tribute prior to the bout as 10 bells tolled in honor of the city's native Ricky Hatton, who died at age 46 in last month.
It was the first major televised match card in the urban center since Hatton's passing. As the last bell ended, the silence transitioned to a roar of "there's only one Hatton".
The competition during the bout equaled the emotion. Arthur was floored by a significant blow flush on the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged soon, significant bruising appearing under his eye as the duo landed punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
The victor edged the result with judges scorecards of draw, close margin and wider gap, but the two athletes were relentless and full of heart, everything their hero the legend stood for.
