UK light heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a contest where the approaches never quite clicked, with excessive holding spoiling the action.
The officials scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They win," Buatsi said in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought Parker, 31, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
The Champion Earns Triumph in Dull Affair
His career development has stalled in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to edge the better rounds.
The spectators' acceptance decreased during regular grabbing, with the challenger falling to the floor multiple occasions during grappling.
Parker finished the stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Fuels Local Rivalry Contest
In the co-main event, Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester contest to win the European light-heavyweight championship.
The arena grew hushed for a extremely emotional tribute before the fight as 10 bells sounded in memory of the city's native Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell ended, the silence became a roar of "there's just one Hatton".
The action during the bout matched the feeling. Arthur was sent to the canvas by a significant blow squarely on the face in the early round, standing uncertainly to his feet on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was injured quickly, heavy swelling appearing under his eye as the pair landed hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of draw, narrow lead and wider gap, but the two athletes were persistent and brave, exactly what their inspiration Hatton embodied.
