UK light-heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a match where the approaches failed to mesh, with constant grabbing disrupting the fight.
The officials had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," the victor stated in the ring as he was met by boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers felt Parker, thirty-one, deserved the win but the British boxer endured the second defeat of his 28-fight career.
The Champion Claims Victory in Uneventful Affair
The fighter's advancement has slowed in recent times through management issues and considerable time of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter ready to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the more effective stanzas.
The spectators' acceptance wore thin amid frequent holding, with the challenger falling to the floor multiple occasions during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a outstanding local rivalry contest to win the continental belt.
The venue grew hushed for a profoundly moving tribute before the fight as ten bells rang in remembrance of the city's native Hatton, who passed away 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 became a chant of "there's only one Hatton".
The action within the ring matched the emotion. The eventual winner was floored by a significant blow squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red shorts of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, heavy swelling appearing under his face as the pair traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the verdict with official scores of draw, narrow lead and 115-112, but both fighters were determined and brave, everything their hero the legend embodied.
