UK light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a fight where the styles didn't complement each other, with constant grabbing marring the fight.
Two judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest performance but what do champions do? They win," Buatsi stated in the ring as he was met by disapproval from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
Buatsi Claims Triumph in Slow Match
His career development has stalled in the past few years through management issues and extended periods of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers willing to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who operated with a solid defense to barely take the better periods.
The audience's tolerance decreased because of constant clinching, with Parker slipping to the canvas various instances during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. Although, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to win the continental title.
The stadium became quiet for a deeply poignant tribute prior to the bout as a bell ceremony tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound disappeared, the silence gave way to a shout of "only one Hatton".
The action inside the ropes matched the sentiment. Arthur was knocked down by a powerful punch squarely on the jaw in the second round, rising shakily to his position on the eight count.
But the traditional red equipment of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was marked up early, noticeable damage developing under his eye as the pair landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the result with scoring tallies of even score, narrow lead and 115-112, but the two athletes were determined and full of heart, exactly what their hero the legend represented.
