British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a fight where the styles never quite clicked, with constant grabbing disrupting the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They prevail," the victor commented in the arena as he was met by negative reactions from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second defeat of his 28-bout career.
The Champion Claims Win in Dull Encounter
Buatsi's progress has stalled in recent times through management issues and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who utilized a protective stance to edge the better stanzas.
The audience's tolerance diminished amid regular grabbing, with Parker slipping to the canvas various instances during clinches.
Parker ended stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a terrific local rivalry fight to win the European light heavyweight championship.
The venue grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony sounded in memory of the city's native Ricky Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the urban center since Hatton's death. As the last bell disappeared, the silence transitioned to a roar of "there's only one Ricky Hatton".
The action inside the ropes equaled the emotion. The eventual winner was knocked down by a significant blow directly to the jaw in the second round, rising shakily to his footing on the count of eight.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was damaged early, significant bruising developing under his orbital area as the pair landed hard punches.
Rea, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with scoring tallies of draw, narrow lead and wider gap, but each competitor were determined and full of heart, everything their inspiration the legend embodied.
