British light-heavyweight Joshua Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a close ten-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a contest where the approaches never quite clicked, with excessive holding marring the contest.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi stated in the ringside as he was met by 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.
Several boxing experts felt Parker, 31, merited the win but the contender experienced the second setback of his 28-match career.
The Champion Secures Win in Uneventful Encounter
The fighter's advancement has slowed in recent years through management issues and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the better periods.
The audience's tolerance decreased amid constant clinching, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester battle to capture the European light-heavyweight title.
The stadium fell silent for a deeply poignant memorial prior to the bout as a bell ceremony sounded in memory of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised match card in the urban center since Hatton's passing. As the last bell ended, the quiet became a roar of "only one Ricky Hatton".
The action during the bout reflected the feeling. Arthur was floored by a left hook flush on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red attire of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured soon, significant bruising appearing under his face as the fighters exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of even score, close margin and 115-112, but both fighters were persistent and brave, everything their inspiration Hatton represented.
