UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a contest where the approaches didn't complement each other, with frequent clinching spoiling the fight.
The officials had it 96-94 to the winner, with the final judge a even score.
"It wasn't my best display but what defines champions? They succeed," the victor said in the ring as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts believed Parker, 31, merited the victory but the British boxer endured the second loss of his 28-bout career.
Buatsi Secures Victory in Uneventful Affair
His career development has stalled in recent times through promotional disputes and extended periods of lack of fights.
Earlier forecast as the next star of British boxing, he now stands trying to achieve expectations - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers ready to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who utilized a tight guard to narrowly win the cleaner rounds.
The crowd's patience diminished amid frequent holding, with the challenger going down several times during close exchanges.
Parker closed well, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry battle to capture the European light-heavyweight title.
The stadium grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony rang in remembrance of Manchester's own Hatton, who passed away 46 years old in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the silence gave way to a shout of "there's only one Hatton".
The action within the ring reflected the sentiment. Arthur was floored by a powerful punch flush on the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was injured early, noticeable damage appearing under his face as the pair landed hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the decision with official scores of even score, narrow lead and clearer margin, but both fighters were relentless and full of heart, exactly what their idol the legend stood for.
