UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches never quite clicked, with constant grabbing marring the fight.
The scoring judges had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi said in the arena as he was faced disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought Parker, 31, deserved the win but the British boxer endured the second loss of his 28-bout professional journey.
Buatsi Claims Triumph in Uneventful Encounter
His career development has plateaued in recent times through management issues and considerable time of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a last chance - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who operated with a protective stance to narrowly win the cleaner periods.
The audience's tolerance decreased amid constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a excellent all-Manchester battle to secure the European light-heavyweight title.
The stadium fell silent for a profoundly moving memorial prior to the bout as a bell ceremony sounded in memory of local hero Hatton, who passed away aged 46 in September.
It was the first major televised boxing event in the city since Hatton's death. As the final chime disappeared, the stillness gave way to a shout of "there's only one Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the face in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling appearing under his orbital area as the fighters exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur edged the verdict with official scores of draw, close margin and clearer margin, but each competitor were persistent and brave, exactly what their idol Hatton stood for.
