British light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round contest at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a match where the approaches didn't complement each other, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best performance but what do champions do? They win," Buatsi commented in the ringside as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Several boxing experts thought Parker, 31, merited the decision but the British boxer endured the second defeat of his 28-bout career.
The Champion Claims Win in Dull Match
The fighter's advancement has stalled in recent years through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter ready to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to narrowly win the more effective periods.
The audience's tolerance wore thin during frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog closed well, but several stanzas were challenging to assess. However, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester fight to secure the continental belt.
The arena became quiet for a extremely emotional homage prior to the bout as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell disappeared, the quiet gave way to a roar of "there's only one Hatton".
The competition during the bout equaled the sentiment. Arthur was knocked down by a left hook flush on the jaw in the second round, rising shakily to his position on the count of eight.
But the red-colored equipment of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was damaged soon, heavy swelling developing under his eye as the duo exchanged hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor edged the verdict with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were persistent and full of heart, all the qualities their hero the legend embodied.
