UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker found success, connecting with powerful single shots in a fight where the styles never quite clicked, with excessive holding spoiling the contest.
The officials had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They win," the victor said in the arena as he was met by boos from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-bout career.
Buatsi Claims Win in Slow Affair
Buatsi's progress has stalled in recent years through promotional disputes and extended periods of lack of fights.
Once tipped as the future of UK boxing, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter prepared to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who worked behind a protective stance to narrowly win the cleaner rounds.
The spectators' acceptance decreased because of constant clinching, with the challenger going down multiple occasions during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent all-Manchester fight to win the European light-heavyweight belt.
The stadium became quiet for a deeply poignant tribute ahead of the contest as 10 bells rang in memory of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the city since Hatton's death. As the last bell ended, the quiet transitioned to a shout of "there's only one Hatton".
The competition inside the ropes equaled the sentiment. Arthur was knocked down by a significant blow squarely on the chin in the early round, getting up unsteadily to his position on the referee's count.
But the traditional red shorts of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising appearing under his eye as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor edged the result with judges scorecards of even score, narrow lead and clearer margin, but each competitor were persistent and courageous, all the qualities their idol the legend embodied.
