British light heavyweight Joshua Buatsi earned a win yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a match where the techniques never quite clicked, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi said in the ringside as he was greeted with disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, thirty-one, merited the decision but the Derbyshire fighter experienced the second setback of his 28-fight career.
The Champion Secures Triumph in Dull Match
Buatsi's progress has stalled in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who utilized a protective stance to edge the more effective periods.
The audience's tolerance diminished because of constant clinching, with Parker going down several times during grappling.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Arthur beat Bradley Rea by divided decision in a terrific all-Manchester battle to capture the continental championship.
The arena fell silent for a deeply poignant memorial before the fight as 10 bells sounded in memory of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the last bell ended, the silence became a chant of "only one Hatton".
The competition within the ring matched the sentiment. The eventual winner was floored by a powerful punch flush on the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the traditional red shorts of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was damaged soon, significant bruising developing under his eye as the fighters landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor barely secured the verdict with scoring tallies of draw, close margin and 115-112, but the two athletes were relentless and brave, all the qualities their hero Hatton embodied.
