British light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a match where the techniques never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They win," Buatsi said in the ringside as he was faced disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the decision but the contender experienced the second defeat of his 28-match professional journey.
The Champion Earns Win in Slow Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who worked behind a solid defense to edge the cleaner rounds.
The audience's tolerance decreased amid frequent holding, with Parker falling to the floor various instances during close exchanges.
Parker ended stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight belt.
The stadium fell silent for a deeply poignant tribute prior to the bout as a bell ceremony tolled in remembrance of Manchester's own Hatton, who died aged 46 in September.
It was the premier televised boxing event in the urban center since Hatton's death. As the concluding sound faded, the silence gave way to a chant of "only one Hatton".
The fighting within the ring equaled the emotion. Arthur was knocked down by a powerful punch directly to the jaw in the early round, rising shakily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was injured quickly, noticeable damage developing under his eye as the duo exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor edged the decision with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were persistent and brave, exactly what their idol Hatton stood for.
