UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches failed to mesh, with excessive holding marring the fight.
The officials scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi stated in the ringside as he was met by disapproval from the audience.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second loss of his 28-bout professional journey.
The Champion Earns Win in Slow Encounter
The fighter's advancement has stalled in recent years through promotional disputes and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under fresh management.
He opened tentatively, with both competitors prepared to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who utilized a solid defense to narrowly win the more effective periods.
The spectators' acceptance diminished during frequent holding, with Parker going down multiple occasions during close exchanges.
Parker closed well, but multiple periods were hard to judge. However, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester contest to win the continental belt.
The stadium became quiet for a extremely emotional memorial before the fight as 10 bells tolled in memory of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the first major televised match card in the location since Hatton's death. As the final chime ended, the quiet became a chant of "there's only one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was knocked down by a significant blow flush on the jaw in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged early, heavy swelling forming under his eye as the duo traded punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
The victor edged the decision with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were determined and full of heart, everything their idol the legend embodied.
