British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive ten-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the techniques failed to mesh, with excessive holding spoiling the fight.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," Buatsi stated in the ring as he was greeted with negative reactions from the crowd.
The British fighter got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the decision but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Earns Win in Slow Affair
Buatsi's progress has slowed in recent years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter willing to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to narrowly win the more effective periods.
The crowd's patience wore thin during constant clinching, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Motivates All-Manchester Contest
In the co-main event, Arthur overcame Bradley Rea by close verdict in a terrific local rivalry contest to capture the European light heavyweight championship.
The stadium became quiet for a profoundly moving tribute before the fight as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the urban center since Hatton's passing. As the final chime disappeared, the quiet gave way to a roar of "there's just one Ricky Hatton".
The fighting during the bout reflected the emotion. Arthur was floored by a significant blow flush on the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured soon, noticeable damage appearing under his eye as the duo exchanged punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
Arthur edged the verdict with scoring tallies of draw, close margin and 115-112, but the two athletes were determined and full of heart, everything their hero the legend stood for.
