UK light heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close ten-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a contest where the approaches didn't complement each other, with excessive holding marring the action.
Two judges judged it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They win," Buatsi commented in the ring as he was greeted with boos from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
The Champion Secures Win in Slow Encounter
His career development has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who operated with a protective stance to edge the cleaner periods.
The spectators' acceptance decreased because of regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
Parker closed well, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight belt.
The venue fell silent for a deeply poignant tribute before the fight as 10 bells sounded in remembrance of the city's native Ricky Hatton, who died 46 years old in recently.
It was the premier televised match card in the urban center since Hatton's death. As the concluding sound faded, the stillness gave way to a roar of "there's only one Hatton".
The action within the ring reflected the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the initial stage, standing uncertainly to his position on the count of eight.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged early, significant bruising appearing under his eye as the duo traded punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur barely secured the decision with scoring tallies of 114-114, close margin and wider gap, but each competitor were relentless and full of heart, everything their idol Hatton represented.
