UK light heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a competitive 10-round fight at the Manchester venue.
But the challenger found success, connecting with powerful clean blows in a fight where the approaches failed to mesh, with frequent clinching disrupting the action.
Two judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest showing but how do champions respond? They prevail," the victor said in the ringside as he was met by boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts believed Parker, thirty-one, merited the decision but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has stalled in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers ready to overcommit in a tentative opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who operated with a protective stance to narrowly win the better rounds.
The crowd's patience wore thin during frequent holding, with the challenger going down various instances during grappling.
The underdog closed well, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a terrific all-Manchester contest to capture the European light-heavyweight belt.
The venue became quiet for a deeply poignant memorial before the fight as 10 bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime ended, the silence transitioned to a roar of "there's just one Hatton".
The action during the bout matched the sentiment. Arthur was knocked down by a significant blow directly to the jaw in the second round, getting up unsteadily to his position on the count of eight.
But the traditional red attire of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was injured early, significant bruising developing under his face as the duo traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the decision with official scores of draw, narrow lead and wider gap, but the two athletes were relentless and full of heart, everything their hero the legend embodied.
