UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the styles didn't complement each other, with excessive holding disrupting the fight.
The officials had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They succeed," the victor stated in the ringside as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers thought the challenger, 31, deserved the victory but the contender experienced the second defeat of his 28-fight career.
The Champion Secures Victory in Dull Encounter
The fighter's advancement has slowed in recent times through contract problems and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to realize his potential - perhaps in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to take risks in a calculated early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who worked behind a protective stance to edge the cleaner stanzas.
The audience's tolerance wore thin because of constant clinching, with Parker going down multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific local rivalry battle to win the European light heavyweight belt.
The venue fell silent for a extremely emotional memorial prior to the bout as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died 46 years old in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime disappeared, the quiet gave way to a shout of "there's just one Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a powerful punch directly to the chin in the second round, standing uncertainly to his position on the eight count.
But the red-colored attire of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was injured early, noticeable damage forming under his face as the duo traded powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the decision with judges scorecards of draw, 115-113 and wider gap, but the two athletes were persistent and courageous, all the qualities their idol Hatton stood for.
