UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a tight ten-round bout at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the techniques didn't complement each other, with constant grabbing marring the contest.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They succeed," the victor commented in the arena as he was met by disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Secures Triumph in Uneventful Affair
Buatsi's progress has plateaued in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter prepared to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to barely take the more effective periods.
The crowd's patience decreased during regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
Parker closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Homage Inspires All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to capture the European light heavyweight title.
The venue fell silent for a deeply poignant tribute ahead of the contest as ten bells rang in memory of Manchester's own Hatton, who succumbed aged 46 in September.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a roar of "only one Hatton".
The competition during the bout equaled the emotion. The eventual winner was knocked down by a powerful punch flush on the chin in the early round, rising shakily to his footing on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up quickly, heavy swelling forming under his orbital area as the pair exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur edged the verdict with official scores of even score, close margin and wider gap, but the two athletes were relentless and brave, everything their idol the legend embodied.
