British light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a match where the techniques didn't complement each other, with excessive holding disrupting the fight.
Two judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the ringside as he was greeted with boos from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Dull Match
His career development has plateaued in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers ready to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who utilized a protective stance to barely take the better periods.
The spectators' acceptance decreased because of frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding all-Manchester contest to capture the continental title.
The venue fell silent for a deeply poignant memorial prior to the bout as ten bells tolled in honor of the city's native Ricky Hatton, who died at age 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound disappeared, the silence became a chant of "there's just one Hatton".
The fighting inside the ropes matched the feeling. Arthur was sent to the canvas by a powerful punch directly to the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged early, significant bruising appearing under his orbital area as the pair exchanged powerful shots.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the decision with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were persistent and courageous, everything their hero the legend represented.
