British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a contest where the approaches never quite clicked, with constant grabbing marring the fight.
The officials judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They win," Buatsi said in the ring as he was faced disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-fight career.
Buatsi Claims Triumph in Uneventful Match
The fighter's advancement has stalled in recent years through promotional disputes and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to edge the better rounds.
The spectators' acceptance diminished during frequent holding, with Parker going down several times during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry contest to win the continental title.
The arena fell silent for a extremely emotional memorial prior to the bout as 10 bells sounded in honor of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the location since Hatton's death. As the concluding sound ended, the stillness gave way to a shout of "there's only one Ricky Hatton".
The action during the bout reflected the emotion. The eventual winner was sent to the canvas by a left hook directly to the jaw in the second round, standing uncertainly to his position on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged quickly, heavy swelling appearing under his face as the pair landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor edged the result with official scores of even score, 115-113 and wider gap, but both fighters were persistent and full of heart, exactly what their inspiration the legend represented.
