UK light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques failed to mesh, with constant grabbing disrupting the fight.
The scoring judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They succeed," Buatsi stated in the ring as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, should have been awarded the decision but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Earns Win in Uneventful Affair
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who utilized a solid defense to narrowly win the cleaner rounds.
The crowd's patience wore thin during regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog closed well, but many rounds were hard to judge. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a excellent local rivalry battle to capture the European light heavyweight title.
The stadium fell silent for a deeply poignant tribute before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime ended, the quiet gave way to a shout of "there's only one Hatton".
The competition within the ring equaled the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the early round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was damaged soon, noticeable damage developing under his face as the pair landed punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor edged the verdict with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were persistent and courageous, everything their inspiration Hatton embodied.
