UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the techniques didn't complement each other, with constant grabbing marring the action.
The officials scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what do champions do? They succeed," the victor stated in the ringside as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the win but the Derbyshire fighter experienced the second loss of his 28-match career.
The Champion Earns Win in Uneventful Encounter
Buatsi's progress has plateaued in recent times through management issues and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a protective stance to edge the cleaner periods.
The crowd's patience decreased because of regular grabbing, with Parker slipping to the canvas multiple occasions during grappling.
Parker finished the stronger, but several stanzas were difficult to score. However, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a outstanding all-Manchester battle to win the European light heavyweight belt.
The venue fell silent for a extremely emotional memorial before the fight as ten bells rang in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the last bell ended, the silence became a shout of "there's only one Hatton".
The action within the ring equaled the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured early, significant bruising appearing under his eye as the duo landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the decision with official scores of 114-114, close margin and wider gap, but both fighters were relentless and brave, everything their idol Hatton represented.
