UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close ten-round bout at the boxing arena.
But the challenger found success, delivering effective clean blows in a fight where the techniques never quite clicked, with excessive holding spoiling the action.
The scoring judges judged it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest performance but what do champions do? They prevail," the victor said in the ring as he was greeted with boos from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the victory but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
Buatsi Claims Victory in Slow Encounter
His career development has stalled in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a last chance - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the more effective periods.
The crowd's patience wore thin during constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Although, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a excellent all-Manchester battle to win the European light-heavyweight belt.
The arena grew hushed for a deeply poignant tribute prior to the bout as ten bells rang in honor of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the premier televised boxing event in the city since Hatton's passing. As the concluding sound faded, the silence gave way to a chant of "there's only one Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured soon, heavy swelling developing under his face as the duo exchanged hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over the full distance.
Arthur barely secured the verdict with scoring tallies of even score, narrow lead and 115-112, but the two athletes were determined and brave, all the qualities their inspiration Hatton stood for.
