UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close 10-round bout at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a match where the techniques didn't complement each other, with frequent clinching disrupting the contest.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They succeed," Buatsi commented in the arena as he was faced boos from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th career victory.
Several boxing experts felt Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
Buatsi Earns Win in Dull Encounter
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of British boxing, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to take risks in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to edge the better rounds.
The crowd's patience decreased because of constant clinching, with Parker falling to the floor several times during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Inspires All-Manchester Fight
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a excellent local rivalry battle to secure the European light heavyweight title.
The venue grew hushed for a profoundly moving memorial before the fight as ten bells rang in honor of Manchester's own Hatton, who died at age 46 in recently.
It was the premier televised match card in the city since Hatton's passing. As the last bell disappeared, the stillness became a shout of "only one Hatton".
The competition inside the ropes matched the feeling. Arthur was floored by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling appearing under his eye as the pair landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur barely secured the verdict with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were relentless and brave, everything their idol Hatton stood for.
