British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive ten-round bout at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a fight where the approaches never quite clicked, with constant grabbing marring the fight.
Two judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," the victor said in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Several boxing experts felt Parker, thirty-one, deserved the win but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Secures Win in Uneventful Match
His career development has plateaued in recent years through management issues and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers prepared to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who utilized a tight guard to edge the better periods.
The crowd's patience diminished because of frequent holding, with Parker slipping to the canvas various instances during clinches.
Parker closed well, but many rounds were difficult to score. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Motivates Local Rivalry Contest
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a excellent local rivalry contest to win the European light heavyweight belt.
The arena became quiet for a deeply poignant tribute ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the city since Hatton's passing. As the last bell disappeared, the stillness became a shout of "there's only one Hatton".
The fighting within the ring matched the sentiment. Arthur was floored by a significant blow flush on the chin in the early round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged early, noticeable damage developing under his face as the duo traded punishing blows.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur edged the result with scoring tallies of even score, narrow lead and wider gap, but both fighters were relentless and full of heart, everything their inspiration the legend stood for.
