British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight 10-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the techniques failed to mesh, with frequent clinching spoiling the action.
The officials had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what do champions do? They win," the victor said in the arena as he was faced negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought the challenger, thirty-one, merited the decision but the Derbyshire fighter endured the second defeat of his 28-fight career.
Buatsi Secures Triumph in Slow Match
The fighter's advancement has stalled in recent years through contract problems and extended periods of lack of fights.
Previously predicted as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors ready to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who worked behind a solid defense to narrowly win the cleaner stanzas.
The crowd's patience wore thin because of constant clinching, with Parker going down various instances during grappling.
The underdog closed well, but many rounds were challenging to assess. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a excellent local rivalry battle to capture the European light heavyweight belt.
The stadium fell silent for a extremely emotional tribute before the fight as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound disappeared, the silence became a shout of "there's just one Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was floored by a left hook directly to the jaw in the initial stage, rising shakily to his position on the count of eight.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was injured quickly, noticeable damage developing under his orbital area as the duo traded powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of draw, close margin and clearer margin, but the two athletes were relentless and full of heart, everything their idol Hatton stood for.
