UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round bout 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 spoiling the fight.
Two judges had it 96-94 to the winner, with the final judge a even score.
"It wasn't my finest display but what defines champions? They prevail," the victor stated in the ring as he was met by negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Various analysts thought Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
Buatsi Secures Win in Dull Match
His career development has stalled in recent years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who utilized a solid defense to narrowly win the cleaner rounds.
The crowd's patience diminished amid frequent holding, with the challenger falling to the floor multiple occasions during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Homage Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a terrific all-Manchester battle to win the European light-heavyweight belt.
The stadium fell silent for a profoundly moving memorial before the fight as 10 bells sounded in honor of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the city since Hatton's death. As the final chime disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The competition within the ring equaled the feeling. The eventual winner was knocked down by a significant blow directly to the face in the second round, getting up unsteadily to his feet on the eight count.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured quickly, heavy swelling developing under his orbital area as the pair traded punishing blows.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor barely secured the verdict with official scores of draw, 115-113 and clearer margin, but the two athletes were determined and courageous, everything their idol the legend represented.
