UK light heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a match where the styles didn't complement each other, with constant grabbing marring the contest.
Two judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but what do champions do? They win," the victor said in the ring as he was greeted with disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, merited the win but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
Buatsi Claims Win in Dull Match
Buatsi's progress has stalled in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter prepared to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who operated with a solid defense to edge the cleaner rounds.
The crowd's patience diminished because of regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester contest to capture the continental belt.
The venue became quiet for a deeply poignant memorial prior to the bout as 10 bells rang in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the silence transitioned to a chant of "there's only one Ricky Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the face in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was injured early, significant bruising appearing under his face as the duo landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over the full distance.
Arthur barely secured the decision with official scores of 114-114, close margin and 115-112, but both fighters were relentless and courageous, all the qualities their inspiration the legend stood for.
