UK light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker found success, landing some telling individual punches in a contest where the styles didn't complement each other, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They succeed," the victor commented in the ringside as he was faced boos from the crowd.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers thought the challenger, thirty-one, deserved the decision but the British boxer experienced the second setback of his 28-fight career.
Buatsi Earns Triumph in Slow Encounter
The fighter's advancement has slowed in the past few years through contract problems and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now stands trying to achieve expectations - maybe in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who utilized a tight guard to barely take the more effective rounds.
The audience's tolerance diminished amid frequent holding, with Parker going down several times during grappling.
Parker ended stronger, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester fight to secure the continental championship.
The arena fell silent for a profoundly moving tribute prior to the bout as a bell ceremony tolled in honor of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the concluding sound faded, the quiet became a roar of "there's just one Hatton".
The competition inside the ropes equaled the feeling. Arthur was floored by a powerful punch squarely on the jaw in the initial stage, rising shakily to his footing on the eight count.
But the traditional red equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was damaged soon, noticeable damage forming under his orbital area as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but both fighters were relentless and full of heart, exactly what their hero Hatton stood for.
