British light heavyweight Joshua Buatsi earned a win yet underwhelming start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close ten-round bout at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a match where the approaches never quite clicked, with frequent clinching marring the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," the victor said in the ring as he was greeted with boos from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the decision but the contender endured the second loss of his 28-match professional journey.
Buatsi Earns Victory in Slow Affair
Buatsi's progress has plateaued in recent years through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to barely take the better stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger falling to the floor several times during grappling.
Parker ended stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Motivates All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to capture the continental title.
The stadium fell silent for a profoundly moving memorial before the fight as a bell ceremony sounded in memory of local hero Hatton, who died 46 years old in recently.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound ended, the quiet gave way to a roar of "only one Hatton".
The fighting during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, noticeable damage developing under his orbital area as the duo traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur narrowly won the result with official scores of draw, 115-113 and 115-112, but each competitor were relentless and brave, exactly what their hero Hatton represented.
