British light-heavyweight Joshua Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round fight at the boxing arena.
But Parker found success, connecting with powerful single shots in a contest where the techniques never quite clicked, with frequent clinching spoiling the action.
The officials scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They prevail," Buatsi said in the ring as he was greeted with negative reactions from the spectators.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts believed the challenger, 31, deserved the decision but the British boxer suffered the second setback of his 28-match career.
The Champion Claims Triumph in Uneventful Match
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who operated with a tight guard to barely take the better periods.
The crowd's patience wore thin because of frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to secure the European light heavyweight championship.
The venue grew hushed for a profoundly moving homage before the fight as 10 bells rang in honor of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime disappeared, the silence transitioned to a roar of "only one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was floored by a powerful punch directly to the jaw in the early round, rising shakily to his position on the referee's count.
But the red-colored attire of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was damaged early, heavy swelling developing under his face as the pair traded powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor edged the verdict with judges scorecards of 114-114, close margin and clearer margin, but both fighters were persistent and brave, everything their inspiration the legend embodied.
