UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a match where the approaches didn't complement each other, with constant grabbing spoiling the contest.
The officials scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," the victor said in the ringside as he was greeted with disapproval from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th career victory.
Various analysts thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second defeat of his 28-fight professional journey.
The Champion Claims Win in Dull Affair
The fighter's advancement has plateaued in recent times through promotional disputes and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with the two boxers prepared to overcommit in a tentative initial rounds.
Parker, whose single earlier setback 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 light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to edge the better periods.
The audience's tolerance decreased amid regular grabbing, with Parker going down various instances during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to secure the continental belt.
The stadium became quiet for a deeply poignant tribute prior to the bout as 10 bells tolled in remembrance of the city's native Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the location since Hatton's passing. As the final chime ended, the stillness became a shout of "only one Hatton".
The action inside the ropes matched the emotion. Arthur was sent to the canvas by a significant blow squarely on the face in the early round, getting up unsteadily to his footing on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up early, noticeable damage forming under his eye as the fighters exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor barely secured the result with official scores of even score, 115-113 and wider gap, but both fighters were determined and brave, everything their idol the legend represented.
