British light-heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with frequent clinching spoiling the fight.
Two judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest display but what do champions do? They prevail," the victor said in the arena as he was faced boos from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers believed the challenger, thirty-one, merited the victory but the British boxer endured the second loss of his 28-fight career.
The Champion Claims Triumph in Uneventful Affair
The fighter's advancement has plateaued in recent years through management issues and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective periods.
The audience's tolerance wore thin because of constant clinching, with Parker going down multiple occasions during clinches.
Parker closed well, but many rounds were hard to judge. However, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester battle to capture the continental title.
The arena became quiet for a extremely emotional homage ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound faded, the silence gave way to a shout of "there's only one Hatton".
The fighting during the bout matched the feeling. Arthur was knocked down by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged early, significant bruising forming under his orbital area as the pair landed powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
The victor barely secured the verdict with judges scorecards of 114-114, close margin and clearer margin, but each competitor were determined and full of heart, all the qualities their hero Hatton stood for.
