UK light heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques didn't complement each other, with frequent clinching spoiling the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but what do champions do? They win," the victor stated in the ringside as he was greeted with disapproval from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the British boxer suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Dull Affair
The fighter's advancement has slowed in recent times through contract problems and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who utilized a protective stance to narrowly win the better stanzas.
The crowd's patience diminished during regular grabbing, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Hatton Memorial Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester contest to secure the continental championship.
The venue became quiet for a extremely emotional homage before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the urban center since the passing of Hatton. As the concluding sound disappeared, the stillness became a chant of "there's only one Hatton".
The action within the ring matched the feeling. Arthur was floored by a left hook directly to the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured early, noticeable damage forming under his face as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
The victor edged the result with official scores of even score, 115-113 and 115-112, but the two athletes were persistent and courageous, everything their inspiration Hatton represented.
