UK light heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a match where the styles didn't complement each other, with constant grabbing marring the fight.
The officials judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They win," Buatsi stated in the arena as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the decision but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
The Champion Claims Triumph in Dull Match
The fighter's advancement has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand in the contest, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the more effective stanzas.
The spectators' acceptance decreased during frequent holding, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but multiple periods were difficult to score. Although, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light-heavyweight championship.
The stadium fell silent for a extremely emotional tribute before the fight as 10 bells tolled in honor of the city's native Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound faded, the silence transitioned to a shout of "there's just one Hatton".
The fighting during the bout reflected the emotion. The eventual winner was knocked down by a significant blow directly to the face in the initial stage, rising shakily to his feet on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling developing under his face as the duo landed powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur edged the verdict with scoring tallies of even score, narrow lead and 115-112, but each competitor were persistent and full of heart, all the qualities their idol the legend stood for.
