British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the styles never quite clicked, with constant grabbing spoiling the contest.
The officials judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the ringside as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the win but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Earns Triumph in Uneventful Match
His career development has slowed in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors ready to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who utilized a solid defense to edge the better rounds.
The spectators' acceptance decreased amid constant clinching, with Parker going down several times during close exchanges.
The underdog ended stronger, but multiple periods were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to capture the continental championship.
The venue grew hushed for a profoundly moving memorial before the fight as 10 bells rang in remembrance of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the last bell ended, the silence transitioned to a chant of "there's only one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was sent to the canvas by a significant blow directly to the face in the second round, rising shakily to his position on the eight count.
But the red-colored equipment of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling forming under his face as the pair exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur barely secured the verdict with scoring tallies of draw, narrow lead and wider gap, but each competitor were persistent and brave, everything their inspiration Hatton represented.
