UK light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a match where the styles never quite clicked, with frequent clinching disrupting the contest.
The scoring judges had it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," Buatsi stated in the ring as he was met by boos from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, merited the victory but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
The Champion Secures Win in Uneventful Affair
The fighter's advancement has slowed in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with both competitors ready to overcommit in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to barely take the cleaner rounds.
The spectators' acceptance diminished because of constant clinching, with Parker slipping to the canvas several times during grappling.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester fight to secure the continental belt.
The arena grew hushed for a profoundly moving homage before the fight as 10 bells rang in honor of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the concluding sound faded, the silence transitioned to a chant of "only one Ricky Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a significant blow flush on the face in the initial stage, rising shakily to his footing on the eight count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage forming under his orbital area as the duo landed hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the decision with scoring tallies of 114-114, close margin and clearer margin, but the two athletes were persistent and courageous, everything their inspiration the legend represented.
