UK light heavyweight Joshua Buatsi earned a win yet disappointing debut 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 judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a contest where the approaches never quite clicked, with constant grabbing disrupting the contest.
Two judges judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi commented in the arena as he was faced boos from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second loss of his 28-fight professional journey.
The Champion Secures Victory in Slow Match
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under fresh management.
He started carefully, with the two boxers ready to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the cleaner stanzas.
The spectators' acceptance decreased because of regular grabbing, with Parker falling to the floor multiple occasions during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester fight to win the continental championship.
The venue fell silent for a deeply poignant homage before the fight as 10 bells tolled in remembrance of the city's native Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the silence gave way to a shout of "there's only one Hatton".
The competition within the ring matched the feeling. Arthur was floored by a left hook flush on the jaw in the second round, rising shakily to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was injured early, heavy swelling appearing under his orbital area as the pair landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were relentless and courageous, exactly what their inspiration the legend represented.
