UK light-heavyweight Joshua Buatsi made a winning yet underwhelming beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close ten-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a fight where the techniques failed to mesh, with frequent clinching spoiling the fight.
The scoring judges scored it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi said in the ringside as he was faced disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, 31, merited the decision but the British boxer endured the second setback of his 28-fight career.
Buatsi Secures Win in Slow Affair
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second appearance at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a protective stance to barely take the better rounds.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. However, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent local rivalry contest to secure the continental championship.
The arena fell silent for a extremely emotional tribute prior to the bout as a bell ceremony rang in memory of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the final chime ended, the stillness became a shout of "there's just one Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a left hook squarely on the chin in the initial stage, standing uncertainly to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged soon, heavy swelling forming under his face as the pair traded punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the decision with judges scorecards of even score, narrow lead and clearer margin, but each competitor were relentless and courageous, all the qualities their idol the legend stood for.
