UK light-heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive ten-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a contest where the styles didn't complement each other, with constant grabbing spoiling the fight.
Two judges scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They prevail," the victor commented in the ringside as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, should have been awarded the decision but the British boxer experienced the second defeat of his 28-fight professional journey.
Buatsi Secures Win in Slow Affair
His career development has slowed in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey 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 bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to edge the more effective periods.
The audience's tolerance diminished because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker closed well, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester fight to secure the continental belt.
The venue became quiet for a deeply poignant memorial before the fight as ten bells rang in remembrance of the city's native Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime ended, the quiet transitioned to a chant of "there's only one Hatton".
The action during the bout reflected the feeling. Arthur was floored by a significant blow directly to the face in the initial stage, getting up unsteadily to his position on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged early, significant bruising developing under his face as the pair traded punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the result with official scores of even score, 115-113 and wider gap, but each competitor were determined and full of heart, everything their inspiration the legend represented.
