British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a contest where the techniques didn't complement each other, with excessive holding spoiling the action.
The scoring judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the arena as he was faced boos from the crowd.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts felt the challenger, 31, should have been awarded the win but the contender experienced the second setback of his 28-match professional journey.
The Champion Secures Triumph in Slow Match
His career development has stalled in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He started carefully, with both competitors willing to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to edge the cleaner stanzas.
The spectators' acceptance wore thin during frequent holding, with Parker going down multiple occasions during clinches.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a terrific all-Manchester battle to secure the European light heavyweight title.
The stadium fell silent for a profoundly moving memorial prior to the bout as 10 bells rang in memory of the city's native Ricky Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's passing. As the concluding sound faded, the stillness gave way to a shout of "there's just one Ricky Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the face in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was injured soon, significant bruising appearing under his eye as the pair traded powerful shots.
The challenger, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur barely secured the result with scoring tallies of draw, close margin and clearer margin, but the two athletes were relentless and brave, exactly what their idol the legend represented.
