British light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close ten-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a contest where the approaches never quite clicked, with frequent clinching marring the fight.
The scoring judges scored it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They succeed," Buatsi said in the ring as he was met by negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts believed the challenger, 31, should have been awarded the victory but the British boxer endured the second loss of his 28-fight career.
The Champion Claims Victory in Dull Affair
His career development has plateaued in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors ready to take risks in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to narrowly win the better periods.
The crowd's patience decreased amid regular grabbing, with Parker going down multiple occasions during clinches.
Parker closed well, but several stanzas were challenging to assess. However, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester contest to win the continental belt.
The arena grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony tolled in memory of Manchester's own Hatton, who succumbed 46 years old in September.
It was the premier televised fight night in the city since Hatton's death. As the last bell faded, the stillness became a chant of "there's just one Ricky Hatton".
The fighting during the bout matched the emotion. Arthur was sent to the canvas by a left hook directly to the jaw in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up early, heavy swelling developing under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the verdict with judges scorecards of 114-114, close margin and wider gap, but both fighters were determined and courageous, everything their hero the legend represented.
