UK light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close ten-round contest at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a contest where the styles didn't complement each other, with constant grabbing marring the contest.
The officials had it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what do champions do? They succeed," Buatsi stated in the ringside as he was met by disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Various analysts felt Parker, 31, merited the decision but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
Buatsi Claims Triumph in Slow Affair
Buatsi's progress has stalled in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to achieve expectations - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter ready to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to barely take the more effective stanzas.
The audience's tolerance wore thin amid regular grabbing, with Parker going down several times during close exchanges.
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, giving Buatsi just one round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur defeated Rea by split-decision in a outstanding all-Manchester fight to secure the European light heavyweight title.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony sounded in honor of Manchester's own Hatton, who died 46 years old in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a left hook flush on the jaw in the second round, getting up unsteadily to his feet on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was marked up quickly, noticeable damage appearing under his orbital area as the pair exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the decision with judges scorecards of even score, narrow lead and wider gap, but the two athletes were persistent and full of heart, everything their hero the legend represented.
