UK light heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the techniques failed to mesh, with constant grabbing disrupting the action.
The officials scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best performance but what do champions do? They prevail," the victor said in the arena as he was faced disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers felt Parker, thirty-one, merited the win but the contender suffered the second defeat of his 28-bout career.
Buatsi Claims Victory in Dull Match
Buatsi's progress has plateaued in the past few years through contract problems and long spells of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who utilized a tight guard to edge the better rounds.
The crowd's patience wore thin during regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor progresses, but with the presence of power puncher David Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light heavyweight belt.
The stadium became quiet for a extremely emotional tribute ahead of the contest as a bell ceremony rang in remembrance of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the stillness gave way to a shout of "there's only one Hatton".
The competition during the bout matched the emotion. Arthur was sent to the canvas by a powerful punch squarely on the face in the initial stage, standing uncertainly to his position on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising forming under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur edged the decision with official scores of draw, close margin and 115-112, but the two athletes were relentless and brave, all the qualities their inspiration Hatton embodied.
