British light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a match where the techniques didn't complement each other, with constant grabbing marring the action.
The officials had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," the victor stated in the ringside as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Several boxing experts felt Parker, thirty-one, merited the win but the British boxer endured the second defeat of his 28-fight career.
His career development has stalled in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under recent support.
He began cautiously, with both competitors willing to take risks in a calculated early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to edge the cleaner rounds.
The spectators' acceptance decreased during constant clinching, with the challenger slipping to the canvas several times during grappling.
Parker closed well, but several stanzas were hard to judge. However, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent local rivalry fight to secure the European light-heavyweight belt.
The venue fell silent for a extremely emotional tribute before the fight as a bell ceremony tolled in memory of local hero Ricky Hatton, who died at age 46 in recently.
It was the first major televised match card in the city since Hatton's death. As the concluding sound disappeared, the silence gave way to a chant of "there's just one Ricky Hatton".
The action during the bout reflected the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged quickly, significant bruising appearing under his eye as the duo traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
The victor edged the verdict with official scores of 114-114, narrow lead and clearer margin, but the two athletes were determined and full of heart, everything their inspiration Hatton stood for.
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