UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close 10-round fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a fight where the techniques didn't complement each other, with constant grabbing disrupting the action.
The officials had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ring as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-match career.
Buatsi Secures Triumph in Dull Encounter
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to edge the cleaner periods.
The spectators' acceptance diminished amid constant clinching, with the challenger falling to the floor various instances during clinches.
Parker ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Motivates Manchester Derby Fight
In the co-main event, Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to capture the European light-heavyweight championship.
The venue fell silent for a deeply poignant tribute prior to the bout as a bell ceremony sounded in remembrance of local hero Hatton, who died at age 46 in recently.
It was the first major televised fight night in the location since Hatton's death. As the final chime faded, the stillness transitioned to a chant of "only one Ricky Hatton".
The fighting during the bout reflected the emotion. Arthur was floored by a significant blow directly to the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged soon, heavy swelling developing under his orbital area as the duo traded powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
The victor narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but both fighters were relentless and full of heart, everything their idol the legend stood for.
