British light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round bout at the boxing arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the techniques never quite clicked, with constant grabbing spoiling the fight.
The scoring judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," the victor commented in the ring as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts felt Parker, thirty-one, merited the victory but the British boxer endured the second loss of his 28-bout career.
The Champion Claims Triumph in Dull Affair
Buatsi's progress has plateaued in the past few years through management issues and long spells of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He began cautiously, with the two boxers willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who operated with a protective stance to barely take the cleaner rounds.
The audience's tolerance decreased during constant clinching, with the challenger going down several times during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. However, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester contest to secure the continental championship.
The venue grew hushed for a profoundly moving homage prior to the bout as 10 bells rang in honor of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the premier televised match card in the city since Hatton's death. As the last bell disappeared, the stillness became a roar of "there's only one Hatton".
The fighting inside the ropes matched the emotion. Arthur was knocked down by a significant blow squarely on the face in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored attire of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was damaged early, noticeable damage developing under his face as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the decision with official scores of draw, 115-113 and 115-112, but each competitor were persistent and full of heart, everything their idol Hatton embodied.
