British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a match where the approaches never quite clicked, with constant grabbing spoiling the contest.
Two judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the ringside as he was met by negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers felt Parker, 31, merited the decision but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
The Champion Earns Win in Slow Match
His career development has stalled in recent times through management issues and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors ready to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who utilized a solid defense to barely take the better rounds.
The crowd's patience decreased during constant clinching, with the challenger going down several times during grappling.
Parker finished the stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the secondary fight, Arthur defeated Rea by split-decision in a excellent local rivalry battle to secure the European light heavyweight belt.
The stadium grew hushed for a profoundly moving homage before the fight as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell faded, the stillness transitioned to a roar of "there's just one Hatton".
The action inside the ropes equaled the feeling. Arthur was floored by a significant blow directly to the jaw in the second round, rising shakily to his position on the count of eight.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up early, noticeable damage forming under his eye as the pair exchanged powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the action hardly diminished over 12 rounds.
The victor edged the result with judges scorecards of 114-114, narrow lead and clearer margin, but the two athletes were persistent and full of heart, everything their idol Hatton embodied.
