British light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under new promoters with a unanimous decision 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 Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a contest where the styles failed to mesh, with excessive holding spoiling the action.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what do champions do? They win," the victor stated in the arena as he was faced boos from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, 31, deserved the decision but the contender endured the second defeat of his 28-fight career.
The Champion Earns Victory in Dull Match
His career development has stalled in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under fresh management.
He opened tentatively, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to edge the more effective stanzas.
The crowd's patience decreased because of regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Inspires Manchester Derby Battle
In the co-main event, Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to secure the continental championship.
The stadium fell silent for a profoundly moving tribute before the fight as a bell ceremony sounded in honor of Manchester's own Hatton, who died at age 46 in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the last bell ended, the quiet transitioned to a roar of "there's just one Hatton".
The competition within the ring reflected the emotion. Arthur was sent to the canvas by a significant blow flush on the face in the initial stage, standing uncertainly to his feet on the eight count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was injured soon, significant bruising appearing under his eye as the duo landed hard punches.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
Arthur barely secured the result with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were determined and brave, all the qualities their idol the legend embodied.
