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 appeared to earn him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker landed significant shots, landing some telling individual punches in a contest where the approaches didn't complement each other, with excessive holding disrupting the fight.
The officials scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest showing but what defines champions? They prevail," the victor stated in the ringside as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt Parker, 31, merited the decision but the contender experienced the second loss of his 28-fight professional journey.
The Champion Earns Win in Uneventful Encounter
His career development has plateaued in recent times through management issues and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter willing to take risks in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance decreased because of regular grabbing, with Parker going down multiple occasions during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the continental title.
The arena fell silent for a deeply poignant tribute prior to the bout as a bell ceremony rang in remembrance of the city's native Hatton, who died aged 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the last bell ended, the quiet became a roar of "there's only one Hatton".
The competition within the ring reflected the emotion. The eventual winner was knocked down by a significant blow flush on the chin in the second round, standing uncertainly to his position on the eight count.
But the traditional red shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was damaged early, noticeable damage forming under his face as the duo exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor edged the decision with official scores of 114-114, close margin and clearer margin, but each competitor were relentless and courageous, everything their idol Hatton stood for.
