UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round contest at the boxing arena.
But Parker found success, delivering effective clean blows in a fight where the approaches failed to mesh, with excessive holding spoiling the fight.
The officials scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest performance but how do champions respond? They succeed," the victor commented in the ringside as he was faced negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, merited the victory but the Derbyshire fighter suffered the second defeat of his 28-match career.
The Champion Secures Win in Uneventful Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He started carefully, with neither fighter willing to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to barely take the cleaner periods.
The crowd's patience decreased during frequent holding, with Parker slipping to the canvas several times during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a excellent local rivalry fight to secure the continental title.
The arena fell silent for a profoundly moving homage prior to the bout as 10 bells tolled in memory of the city's native Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the location since Hatton's passing. As the final chime disappeared, the stillness gave way to a shout of "there's only one Hatton".
The action during the bout equaled the feeling. The eventual winner was sent to the canvas by a left hook directly to the jaw in the early round, getting up unsteadily to his position on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage forming under his eye as the fighters traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and brave, everything their hero the legend represented.
