UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a match where the styles didn't complement each other, with excessive holding marring the fight.
Two judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They win," Buatsi stated in the ring as he was faced negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Various analysts believed Parker, 31, deserved the decision but the contender experienced the second loss of his 28-fight professional journey.
The Champion Secures Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to overcommit in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was participating in his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to edge the cleaner periods.
The spectators' acceptance diminished amid frequent holding, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. Although, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur defeated Rea by split-decision in a outstanding all-Manchester contest to secure the European light heavyweight belt.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell faded, the silence transitioned to a shout of "only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was sent to the canvas by a left hook flush on the face in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was marked up soon, noticeable damage forming under his orbital area as the duo traded punishing blows.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur edged the result with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were determined and courageous, all the qualities their inspiration Hatton represented.
