British light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a match where the techniques failed to mesh, with excessive holding spoiling the contest.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," Buatsi stated in the arena as he was met by boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to secure a 20th career victory.
Some ringside observers believed Parker, 31, merited the win but the contender experienced the second setback of his 28-match professional journey.
Buatsi Earns Win in Dull Encounter
The fighter's advancement has stalled in recent times through promotional disputes and long spells of inactivity.
Once tipped as the next star of British boxing, he now stands trying to realize his potential - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to edge the better rounds.
The spectators' acceptance wore thin during regular grabbing, with the challenger going down various instances during grappling.
Parker closed well, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to capture the European light-heavyweight title.
The arena became quiet for a deeply poignant homage before the fight as 10 bells tolled in remembrance of the city's native Hatton, who died at age 46 in recently.
It was the first major televised match card in the location since Hatton's death. As the last bell ended, the silence transitioned to a roar of "there's only one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the face in the early round, standing uncertainly to his footing on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up quickly, significant bruising appearing under his orbital area as the duo exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur barely secured the result with official scores of 114-114, 115-113 and wider gap, but the two athletes were determined and brave, everything their hero the legend stood for.
