British light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the approaches failed to mesh, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best display but what defines champions? They prevail," the victor said in the arena as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the win but the British boxer endured the second defeat of his 28-bout professional journey.
Buatsi Earns Triumph in Dull Encounter
The fighter's advancement has slowed in the past few years through management issues and extended periods of lack of fights.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter willing to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who utilized a solid defense to edge the better stanzas.
The audience's tolerance diminished because of regular grabbing, with the challenger going down several times during grappling.
Parker ended stronger, but several stanzas were difficult to score. However, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Arthur beat Rea by split-decision in a excellent local rivalry contest to secure the European light-heavyweight title.
The arena fell silent for a deeply poignant tribute prior to the bout as 10 bells rang in memory of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised boxing event in the urban center since Hatton's death. As the final chime disappeared, the quiet became a shout of "only one Ricky Hatton".
The fighting inside the ropes matched the feeling. The eventual winner was sent to the canvas by a left hook flush on the chin in the second round, rising shakily to his footing on the count of eight.
But the traditional red equipment of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was injured early, heavy swelling developing under his eye as the duo traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor edged the verdict with official scores of even score, narrow lead and clearer margin, but the two athletes were determined and brave, everything their hero the legend embodied.
