UK light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a match where the styles never quite clicked, with frequent clinching disrupting the action.
The officials scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi commented in the arena as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Some ringside observers felt Parker, 31, should have been awarded the victory but the contender suffered the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Slow Affair
The fighter's advancement has slowed in recent years through management issues and considerable time of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter ready to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The audience's tolerance wore thin amid constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester contest to secure the European light heavyweight championship.
The stadium fell silent for a extremely emotional tribute prior to the bout as ten bells tolled in honor of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound ended, the silence transitioned to a chant of "there's only one Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a significant blow flush on the face in the second round, rising shakily to his footing on the eight count.
But the traditional red attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged early, significant bruising forming under his face as the fighters traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the intensity seldom decreased over twelve rounds.
The victor edged the result with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were relentless and courageous, all the qualities their idol Hatton represented.
