British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a close ten-round bout at the boxing arena.
But Parker found success, connecting with powerful clean blows in a fight where the styles never quite clicked, with frequent clinching disrupting the fight.
Two judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They win," Buatsi commented in the ring as he was faced negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts believed the challenger, thirty-one, deserved the win but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Secures Triumph in Slow Encounter
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to take risks in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The spectators' acceptance diminished amid constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester fight to secure the European light-heavyweight championship.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the concluding sound ended, the silence transitioned to a roar of "there's only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, getting up unsteadily to his position on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was injured soon, heavy swelling developing under his orbital area as the pair traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur narrowly won the verdict with official scores of 114-114, close margin and 115-112, but both fighters were relentless and courageous, exactly what their inspiration Hatton represented.
