UK light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful individual punches in a match where the styles never quite clicked, with excessive holding spoiling the contest.
Two judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor said in the ringside as he was met by disapproval from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, merited the decision but the British boxer suffered the second loss of his 28-fight professional journey.
Buatsi's progress has stalled in recent times through management issues and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a last chance - under fresh management.
He started carefully, with neither fighter willing to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who worked behind a solid defense to narrowly win the better rounds.
The spectators' acceptance wore thin because of frequent holding, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester battle to win the European light heavyweight title.
The arena fell silent for a deeply poignant memorial prior to the bout as 10 bells tolled in remembrance of local hero Ricky Hatton, who died aged 46 in last month.
It was the first major televised match card in the city since Hatton's passing. As the concluding sound faded, the silence gave way to a shout of "only one Ricky Hatton".
The competition inside the ropes equaled the sentiment. Arthur was knocked down by a significant blow squarely on the face in the second round, getting up unsteadily to his position on the eight count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage developing under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur barely secured the result with official scores of 114-114, narrow lead and 115-112, but the two athletes were relentless and full of heart, everything their idol Hatton embodied.
A passionate gaming enthusiast and expert in online slots, sharing insights and strategies to help players win big.