British light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round bout at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the techniques failed to mesh, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi commented in the arena as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts thought the challenger, 31, deserved the decision but the British boxer endured the second defeat of his 28-fight career.
Buatsi Claims Triumph in Uneventful Match
His career development has stalled in the past few years through management issues and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He started carefully, with the two boxers prepared to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the more effective stanzas.
The crowd's patience wore thin during regular grabbing, with Parker going down multiple occasions during clinches.
Parker closed well, but many rounds were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding local rivalry battle to capture the European light-heavyweight championship.
The stadium grew hushed for a deeply poignant memorial prior to the bout as a bell ceremony sounded in memory of local hero Hatton, who died 46 years old in recently.
It was the first major televised fight night in the city since Hatton's passing. As the last bell disappeared, the quiet gave way to a shout of "only one Hatton".
The competition during the bout equaled the emotion. Arthur was knocked down by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, heavy swelling forming under his orbital area as the fighters exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor edged the decision with scoring tallies of even score, narrow lead and wider gap, but both fighters were relentless and full of heart, all the qualities their hero the legend stood for.
