UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a close 10-round contest at the Manchester venue.
But the challenger found success, delivering effective single shots in a fight where the approaches failed to mesh, with frequent clinching marring the contest.
Two judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest showing but what defines champions? They prevail," the victor commented in the ringside as he was faced boos from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed the challenger, thirty-one, should have been awarded the decision but the contender endured the second setback of his 28-match career.
The Champion Earns Triumph in Uneventful Match
His career development has slowed in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who utilized a solid defense to narrowly win the cleaner rounds.
The crowd's patience diminished amid regular grabbing, with Parker falling to the floor various instances during clinches.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a terrific local rivalry contest to capture the European light heavyweight belt.
The venue became quiet for a deeply poignant homage prior to the bout as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised match card in the city since Hatton's death. As the concluding sound faded, the quiet gave way to a roar of "there's only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a powerful punch directly to the face in the early round, standing uncertainly to his position on the eight count.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising forming under his eye as the duo traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur barely secured the verdict with official scores of 114-114, narrow lead and clearer margin, but each competitor were relentless and brave, everything their idol Hatton represented.
