UK light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a match where the styles failed to mesh, with frequent clinching disrupting the action.
The officials scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but what defines champions? They succeed," the victor said in the arena as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Various analysts felt the challenger, thirty-one, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
The Champion Secures Win in Dull Match
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who operated with a solid defense to barely take the better rounds.
The crowd's patience decreased during constant clinching, with Parker going down several times during grappling.
Parker ended stronger, but several stanzas were challenging to assess. However, former world champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Homage Motivates All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent local rivalry contest to secure the continental title.
The venue fell silent for a extremely emotional memorial prior to the bout as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the concluding sound faded, the silence became a chant of "only one Hatton".
The competition within the ring reflected the feeling. Arthur was floored by a powerful punch directly to the face in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red shorts of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was damaged quickly, significant bruising developing under his face as the duo traded powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over 12 rounds.
The victor barely secured the result with judges scorecards of even score, 115-113 and 115-112, but each competitor were relentless and courageous, everything their idol the legend represented.
