British light-heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker had his moments, delivering effective single shots in a contest where the approaches failed to mesh, with excessive holding disrupting the contest.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They win," the victor stated in the ringside as he was greeted with disapproval from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to earn a 20th career victory.
Several boxing experts believed Parker, 31, merited the victory but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Uneventful Affair
The fighter's advancement has slowed in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who operated with a solid defense to barely take the more effective rounds.
The crowd's patience wore thin during frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light heavyweight belt.
The stadium became quiet for a extremely emotional homage ahead of the contest as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the city since Hatton's passing. As the final chime disappeared, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The competition during the bout equaled the feeling. The eventual winner was knocked down by a left hook flush on the face in the second round, rising shakily to his footing on the referee's count.
But the red-colored equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up soon, noticeable damage forming under his orbital area as the duo traded punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor barely secured the decision with official scores of draw, narrow lead and wider gap, but the two athletes were relentless and brave, all the qualities their idol the legend represented.
