UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a fight where the techniques never quite clicked, with constant grabbing disrupting the action.
The officials had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest performance but how do champions respond? They succeed," Buatsi commented in the arena as he was met by boos from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the victory but the contender endured the second loss of his 28-match career.
Buatsi Secures Triumph in Dull Match
His career development has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the more effective rounds.
The spectators' acceptance wore thin during constant clinching, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight belt.
The arena fell silent for a extremely emotional homage before the fight as a bell ceremony tolled in remembrance of local hero Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the last bell ended, the silence transitioned to a roar of "there's just one Ricky Hatton".
The action within the ring reflected the feeling. The eventual winner was knocked down by a significant blow flush on the chin in the second round, getting up unsteadily to his footing on the count of eight.
But the red-colored equipment of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was injured quickly, noticeable damage forming under his orbital area as the duo landed hard punches.
Rea, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the decision with judges scorecards of even score, 115-113 and clearer margin, but each competitor were persistent and full of heart, everything their inspiration Hatton represented.
