British light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the styles never quite clicked, with constant grabbing marring the fight.
The officials scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi commented in the ringside as he was faced boos from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts believed the challenger, 31, merited the victory but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
The Champion Claims Win in Dull Affair
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He started carefully, with both competitors prepared to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who worked behind a protective stance to barely take the more effective stanzas.
The crowd's patience decreased because of frequent holding, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester contest to secure the European light heavyweight belt.
The arena fell silent for a profoundly moving memorial ahead of the contest as 10 bells rang in honor of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime disappeared, the silence gave way to a roar of "there's just one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was knocked down by a significant blow directly to the jaw in the second round, rising shakily to his feet on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged quickly, noticeable damage forming under his orbital area as the duo landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the result with scoring tallies of 114-114, close margin and clearer margin, but the two athletes were determined and full of heart, exactly what their inspiration Hatton embodied.
