UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round fight at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the techniques never quite clicked, with frequent clinching marring the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi said in the ring as he was greeted with negative reactions from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Various analysts thought Parker, 31, should have been awarded the decision but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Claims Victory in Slow Affair
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the more effective stanzas.
The crowd's patience wore thin amid frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester contest to secure the European light heavyweight title.
The venue fell silent for a deeply poignant homage before the fight as ten bells rang in memory of the city's native Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound faded, the quiet became a roar of "only one Hatton".
The competition during the bout equaled the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the initial stage, rising shakily to his footing on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged soon, heavy swelling developing under his face as the pair exchanged punishing blows.
Rea, 27, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the decision with scoring tallies of even score, close margin and clearer margin, but both fighters were persistent and courageous, everything their inspiration Hatton represented.
