UK light-heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close ten-round fight at the Manchester venue.
But the challenger had his moments, landing some telling clean blows in a match where the approaches never quite clicked, with excessive holding disrupting the action.
Two judges had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the arena as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts thought Parker, 31, deserved the victory but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Earns Triumph in Uneventful Match
His career development has stalled in recent times through promotional disputes and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He started carefully, with neither fighter prepared to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a protective stance to barely take the cleaner stanzas.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor various instances during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent all-Manchester fight to win the European light-heavyweight championship.
The stadium grew hushed for a profoundly moving tribute ahead of the contest as 10 bells rang in memory of Manchester's own Hatton, who died 46 years old in last month.
It was the first major televised match card in the urban center since Hatton's death. As the final chime faded, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action within the ring reflected the emotion. The eventual winner was floored by a significant blow directly to the face in the second round, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up quickly, heavy swelling developing under his orbital area as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the result with judges scorecards of even score, narrow lead and clearer margin, but each competitor were determined and brave, exactly what their hero Hatton stood for.
