British light heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a match where the styles failed to mesh, with excessive holding marring the action.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," the victor said in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, merited the decision but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Earns Victory in Uneventful Match
Buatsi's progress has stalled in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter willing to take risks in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who utilized a solid defense to narrowly win the cleaner periods.
The crowd's patience decreased because of frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
Parker finished the stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry fight to capture the continental championship.
The venue grew hushed for a deeply poignant tribute prior to the bout as ten bells tolled in remembrance of local hero Hatton, who died 46 years old in September.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime faded, the stillness transitioned to a roar of "only one Hatton".
The fighting within the ring equaled the emotion. Arthur was knocked down by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was damaged quickly, significant bruising appearing under his eye as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were relentless and courageous, all the qualities their idol the legend represented.
