UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close 10-round bout at the boxing arena.
But Parker had his moments, delivering effective clean blows in a fight where the approaches didn't complement each other, with frequent clinching disrupting the fight.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They prevail," the victor stated in the ringside as he was faced boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Some ringside observers felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Secures Victory in Dull Affair
Buatsi's progress has stalled in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to realize his potential - maybe in what appears to be a last chance - under recent support.
He opened tentatively, with the two boxers willing to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who operated with a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance diminished because of frequent holding, with Parker falling to the floor multiple occasions during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to win the continental belt.
The stadium fell silent for a profoundly moving homage before the fight as a bell ceremony sounded in honor of the city's native Hatton, who died aged 46 in last month.
It was the first major televised match card in the urban center since Hatton's death. As the final chime disappeared, the quiet transitioned to a shout of "there's only one Hatton".
The fighting during the bout equaled the emotion. Arthur was knocked down by a powerful punch flush on the face in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged early, heavy swelling forming under his orbital area as the pair exchanged punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur barely secured the decision with official scores of draw, 115-113 and 115-112, but both fighters were relentless and full of heart, all the qualities their inspiration Hatton stood for.
