UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close ten-round fight at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the styles didn't complement each other, with constant grabbing spoiling the action.
Two judges had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They prevail," the victor stated in the ring as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the victory but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Affair
The fighter's advancement has slowed in recent years through contract problems and long spells of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter prepared to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging 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 the favorite, who operated with a solid defense to edge the better stanzas.
The audience's tolerance decreased during frequent holding, with Parker slipping to the canvas several times during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher Benavidez, and Russian champion Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the co-main event, Arthur overcame Rea by divided decision in a outstanding all-Manchester fight to capture the continental title.
The arena became quiet for a extremely emotional tribute before the fight as a bell ceremony rang in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the last bell ended, the silence transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring matched the feeling. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up early, significant bruising developing under his face as the duo exchanged hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the verdict with scoring tallies of draw, close margin and 115-112, but both fighters were persistent and brave, everything their idol Hatton embodied.
