UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a contest where the styles never quite clicked, with constant grabbing spoiling the action.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They win," the victor said in the ringside as he was met by disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the victory but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
The Champion Secures Win in Slow Match
His career development has stalled in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the more effective rounds.
The spectators' acceptance diminished because of regular grabbing, with the challenger going down various instances during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding local rivalry contest to secure the European light-heavyweight title.
The venue became quiet for a profoundly moving memorial ahead of the contest as ten bells rang in honor of the city's native Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the last bell faded, the silence transitioned to a roar of "there's just one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was floored by a significant blow directly to the jaw in the second round, standing uncertainly to his position on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged quickly, noticeable damage appearing under his eye as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
The victor barely secured the verdict with official scores of even score, close margin and wider gap, but the two athletes were persistent and brave, all the qualities their hero the legend embodied.
