British light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the approaches didn't complement each other, with excessive holding marring the action.
Two judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor stated in the ring as he was met by negative reactions from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, deserved the victory but the British boxer experienced the second setback of his 28-fight career.
Buatsi Secures Victory in Uneventful Match
Buatsi's progress has stalled in recent times through management issues and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a cagey initial 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 appearance at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to barely take the cleaner rounds.
The spectators' acceptance decreased because of regular grabbing, with Parker going down multiple occasions during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester battle to secure the European light-heavyweight title.
The arena became quiet for a deeply poignant homage before the fight as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised boxing event in the location since Hatton's death. As the final chime disappeared, the silence transitioned to a shout of "there's only one Hatton".
The fighting within the ring equaled the sentiment. Arthur was floored by a left hook flush on the chin in the second round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising developing under his orbital area as the duo traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor barely secured the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were persistent and courageous, exactly what their hero the legend embodied.
