UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a contest where the styles never quite clicked, with frequent clinching spoiling the contest.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best display but how do champions respond? They succeed," the victor stated in the arena as he was faced boos from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers felt Parker, 31, deserved the victory but the contender experienced the second defeat of his 28-bout career.
The Champion Claims Win in Slow Encounter
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to overcommit in a cagey early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to barely take the more effective rounds.
The spectators' acceptance diminished amid regular grabbing, with the challenger going down several times during grappling.
Parker closed well, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Arthur overcame Rea by split-decision in a excellent all-Manchester battle to capture the European light-heavyweight championship.
The stadium fell silent for a deeply poignant tribute before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the location since the passing of Hatton. As the final chime faded, the stillness became a chant of "there's only one Hatton".
The action within the ring reflected the emotion. Arthur was sent to the canvas by a left hook directly to the chin in the second round, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage appearing under his orbital area as the duo exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over the full distance.
The victor narrowly won the decision with official scores of draw, narrow lead and clearer margin, but the two athletes were persistent and full of heart, everything their inspiration Hatton embodied.
