UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a match where the styles never quite clicked, with frequent clinching disrupting the action.
The officials scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest display but how do champions respond? They win," Buatsi stated in the arena as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the victory but the British boxer endured the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Slow Match
The fighter's advancement has plateaued in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with the two boxers willing to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who utilized a tight guard to narrowly win the cleaner periods.
The spectators' acceptance decreased during regular grabbing, with Parker going down multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to win the European light heavyweight championship.
The arena became quiet for a extremely emotional memorial prior to the bout as a bell ceremony rang in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the final chime faded, the silence gave way to a chant of "only one Hatton".
The fighting during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was marked up soon, significant bruising developing under his orbital area as the duo landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the result with judges scorecards of draw, close margin and wider gap, but each competitor were relentless and courageous, exactly what their inspiration Hatton stood for.
