British light heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a tight 10-round fight at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a match where the styles failed to mesh, with excessive holding marring the fight.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the arena as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second loss of his 28-match career.
The Champion Earns Triumph in Dull Match
Buatsi's progress has stalled in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to narrowly win the cleaner stanzas.
The spectators' acceptance decreased during frequent holding, with Parker falling to the floor several times during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the co-main event, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light heavyweight title.
The venue fell silent for a profoundly moving homage ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime ended, the quiet gave way to a chant of "there's just one Ricky Hatton".
The competition during the bout equaled the feeling. Arthur was floored by a significant blow squarely on the face in the initial stage, standing uncertainly to his position on the referee's count.
But the red-colored attire of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was damaged early, significant bruising developing under his face as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over the full distance.
Arthur edged the result with scoring tallies of draw, narrow lead and 115-112, but each competitor were relentless and brave, everything their idol the legend stood for.
