British light heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a fight where the styles failed to mesh, with excessive holding marring the contest.
The officials had it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor said in the ringside as he was faced disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt Parker, 31, should have been awarded the win but the contender experienced the second loss of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who utilized a solid defense to edge the cleaner rounds.
The spectators' acceptance wore thin amid frequent holding, with Parker going down multiple occasions during grappling.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to win the European light heavyweight belt.
The stadium became quiet for a extremely emotional homage prior to the bout as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who died aged 46 in recently.
It was the first major televised match card in the city since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a roar of "there's only one Hatton".
The fighting inside the ropes reflected the feeling. The eventual winner was knocked down by a powerful punch squarely on the chin in the initial stage, rising shakily to his position on the eight count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was damaged early, significant bruising appearing under his orbital area as the pair landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
The victor barely secured the result with scoring tallies of 114-114, narrow lead and 115-112, but each competitor were persistent and courageous, everything their idol the legend represented.
