UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight 10-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the techniques never quite clicked, with frequent clinching marring the fight.
The officials scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the arena as he was greeted with negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought the challenger, 31, should have been awarded the decision but the Derbyshire fighter experienced the second setback of his 28-bout career.
Buatsi Secures Victory in Uneventful Affair
Buatsi's progress has slowed in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He started carefully, with neither fighter ready to overcommit in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who utilized a solid defense to narrowly win the cleaner stanzas.
The crowd's patience decreased amid frequent holding, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific all-Manchester contest to win the European light heavyweight title.
The arena fell silent for a profoundly moving tribute prior to the bout as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised match card in the urban center since the passing of Hatton. As the concluding sound ended, the quiet gave way to a roar of "there's just one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was floored by a powerful punch squarely on the jaw in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red attire of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was damaged early, heavy swelling appearing under his orbital area as the duo landed punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the verdict with judges scorecards of draw, 115-113 and 115-112, but the two athletes were persistent and full of heart, everything their inspiration Hatton embodied.
