UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a match where the approaches didn't complement each other, with constant grabbing spoiling the action.
The scoring judges judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought Parker, 31, deserved the win but the contender suffered the second setback of his 28-bout career.
The Champion Secures Victory in Uneventful Match
His career development has slowed in the past few years through contract problems and long spells of lack of fights.
Once tipped as the future of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who operated with a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance wore thin during frequent holding, with the challenger slipping to the canvas several times during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by split-decision in a excellent local rivalry battle to capture the continental belt.
The arena became quiet for a deeply poignant tribute prior to the bout as ten bells tolled in memory of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound disappeared, the silence became 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 left hook directly to the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was marked up soon, heavy swelling developing under his face as the fighters traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor barely secured the result with judges scorecards of draw, close margin and clearer margin, but each competitor were relentless and brave, all the qualities their hero the legend stood for.
