UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a close 10-round bout at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches didn't complement each other, with constant grabbing marring the contest.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the ring as he was met by negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought Parker, 31, deserved the decision but the British boxer experienced the second setback of his 28-match career.
Buatsi Earns Triumph in Dull Match
The fighter's advancement has plateaued in recent times through management issues and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to edge the cleaner periods.
The spectators' acceptance diminished during regular grabbing, with the challenger going down various instances during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor progresses, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a excellent all-Manchester battle to win the continental championship.
The stadium fell silent for a extremely emotional tribute prior to the bout as 10 bells tolled in memory of the city's native Hatton, who passed away aged 46 in last month.
It was the premier televised match card in the urban center since the passing of Hatton. As the final chime disappeared, the quiet became a roar of "there's just one Hatton".
The competition within the ring matched the feeling. The eventual winner was sent to the canvas by a left hook flush on the jaw in the early round, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was damaged early, noticeable damage developing under his face as the duo landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over 12 rounds.
The victor edged the decision with judges scorecards of even score, narrow lead and wider gap, but the two athletes were relentless and courageous, everything their hero the legend stood for.
