British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective clean blows in a match where the techniques failed to mesh, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They succeed," the victor stated in the arena as he was met by boos from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts felt the challenger, 31, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-match career.
Buatsi Earns Win in Uneventful Affair
Buatsi's progress has slowed in recent years through management issues and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with the two boxers ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who operated with a protective stance to barely take the more effective rounds.
The audience's tolerance decreased during frequent holding, with Parker falling to the floor various instances during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight belt.
The arena became quiet for a deeply poignant tribute ahead of the contest as ten bells tolled in honor of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the city since the passing of Hatton. As the last bell faded, the silence transitioned to a roar of "there's just one Hatton".
The action inside the ropes equaled the feeling. Arthur was floored by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up soon, significant bruising forming under his orbital area as the fighters traded punishing blows.
Rea, 27, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor narrowly won the result with scoring tallies of 114-114, 115-113 and wider gap, but the two athletes were persistent and courageous, everything their hero the legend embodied.
