UK light heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a contest where the styles didn't complement each other, with constant grabbing spoiling the contest.
The scoring judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi stated in the ring as he was faced negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second loss of his 28-fight career.
Buatsi Secures Triumph in Slow Match
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers prepared to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who operated with a tight guard to barely take the cleaner stanzas.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor several times during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Homage Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester battle to secure the European light heavyweight belt.
The stadium grew hushed for a profoundly moving memorial before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell faded, the stillness became a chant of "there's just one Hatton".
The fighting during the bout reflected the feeling. Arthur was floored by a powerful punch directly to the face in the second round, rising shakily to his feet on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was injured quickly, significant bruising appearing under his face as the pair traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
The victor edged the decision with scoring tallies of even score, narrow lead and wider gap, but each competitor were persistent and brave, exactly what their inspiration Hatton stood for.
