British light-heavyweight Joshua Buatsi made a winning yet disappointing start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a tight ten-round fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a match where the techniques didn't complement each other, with constant grabbing marring the action.
The scoring judges scored it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi said in the ring as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts thought Parker, 31, should have been awarded the decision but the contender suffered the second defeat of his 28-bout career.
The Champion Claims Win in Dull Affair
Buatsi's progress has stalled in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to overcommit in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the cleaner stanzas.
The audience's tolerance wore thin amid constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester fight to secure the European light-heavyweight championship.
The arena became quiet for a deeply poignant tribute ahead of the contest as 10 bells tolled in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the final chime disappeared, the stillness gave way to a roar of "only one Hatton".
The competition during the bout reflected the sentiment. Arthur was knocked down by a significant blow flush on the chin in the second round, standing uncertainly to his footing on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was damaged early, noticeable damage developing under his eye as the pair traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor barely secured the decision with official scores of even score, narrow lead and wider gap, but the two athletes were relentless and courageous, exactly what their idol Hatton embodied.
