UK light-heavyweight Joshua Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches failed to mesh, with constant grabbing marring the contest.
The scoring judges had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest performance but what do champions do? They win," the victor commented in the ringside as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers felt Parker, 31, merited the win but the Derbyshire fighter endured the second setback of his 28-bout career.
The Champion Claims Triumph in Dull Encounter
The fighter's advancement has plateaued in recent times through management issues and considerable time of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers prepared to take risks in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who utilized a tight guard to edge the cleaner periods.
The audience's tolerance diminished because of constant clinching, with Parker slipping to the canvas several times during grappling.
Parker closed well, but many rounds were challenging to assess. Although, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Motivates Local Rivalry Contest
In the supporting bout, Arthur beat Rea by split-decision in a outstanding local rivalry contest to secure the European light-heavyweight belt.
The stadium became quiet for a deeply poignant tribute before the fight as 10 bells tolled in honor of local hero Hatton, who passed away aged 46 in September.
It was the premier televised fight night in the city since Hatton's passing. As the final chime disappeared, the silence gave way to a shout of "there's just one Hatton".
The competition during the bout matched the feeling. Arthur was knocked down by a left hook directly to the chin in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured early, significant bruising forming under his eye as the duo traded powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor edged the result with official scores of draw, narrow lead and clearer margin, but the two athletes were determined and full of heart, everything their idol the legend stood for.
