British light heavyweight Joshua Buatsi made a winning yet underwhelming debut to life 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 close ten-round contest at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a match where the techniques failed to mesh, with excessive holding spoiling the fight.
Two judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They win," Buatsi stated in the ring as he was greeted with negative reactions from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-fight career.
Buatsi Claims Triumph in Slow Affair
Buatsi's progress has stalled in recent times through management issues and considerable time of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking 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, making life awkward for the favorite, who utilized a tight guard to edge the more effective periods.
The spectators' acceptance diminished during frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester contest to capture the continental belt.
The arena grew hushed for a deeply poignant homage before the fight as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime faded, the stillness gave way to a roar of "there's only one Ricky Hatton".
The action within the ring equaled the sentiment. Arthur was knocked down by a significant blow squarely on the chin in the early round, getting up unsteadily to his footing on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured early, significant bruising developing under his orbital area as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the verdict with official scores of 114-114, 115-113 and clearer margin, but each competitor were persistent and full of heart, exactly what their inspiration the legend stood for.
