UK light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a contest where the techniques never quite clicked, with excessive holding marring the fight.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," the victor commented in the ringside as he was met by negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second defeat of his 28-bout career.
Buatsi Secures Win in Slow Match
The fighter's advancement has stalled in recent years through contract problems and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a tight guard to barely take the more effective rounds.
The crowd's patience wore thin amid regular grabbing, with Parker falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by split-decision in a excellent Manchester vs Manchester contest to secure the European light heavyweight championship.
The venue became quiet for a extremely emotional homage before the fight as 10 bells tolled in memory of local hero Hatton, who died 46 years old in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime disappeared, the stillness became a roar of "there's only one Ricky Hatton".
The competition inside the ropes reflected the feeling. Arthur was knocked down by a powerful punch directly to the face in the second round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged soon, significant bruising developing under his face as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over the full distance.
The victor edged the verdict with judges scorecards of even score, narrow lead and wider gap, but each competitor were relentless and courageous, all the qualities their idol the legend represented.
