British light-heavyweight Joshua Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger had his moments, delivering effective single shots in a match where the styles didn't complement each other, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They succeed," the victor said in the ring as he was faced boos from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers believed the challenger, thirty-one, merited the victory but the contender suffered the second setback of his 28-bout career.
Buatsi Earns Win in Slow Affair
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to edge the cleaner rounds.
The spectators' acceptance wore thin amid constant clinching, with the challenger slipping to the canvas several times during close exchanges.
Parker closed well, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific all-Manchester battle to capture the European light heavyweight championship.
The arena fell silent for a extremely emotional tribute prior to the bout as ten bells rang in memory of local hero Hatton, who passed away at age 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness became a shout of "only one Ricky Hatton".
The competition within the ring equaled the emotion. Arthur was sent to the canvas by a powerful punch flush on the jaw in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured soon, noticeable damage developing under his orbital area as the fighters traded punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the action rarely dropped over the full distance.
Arthur barely secured the result with official scores of 114-114, narrow lead and wider gap, but the two athletes were determined and brave, exactly what their inspiration the legend represented.
