British light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But Parker had his moments, delivering effective clean blows in a contest where the styles never quite clicked, with excessive holding marring the fight.
The officials scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest display but what do champions do? They succeed," Buatsi commented in the ring as he was met by negative reactions from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the victory but the Derbyshire fighter experienced the second setback of his 28-match career.
The Champion Secures Victory in Slow Affair
The fighter's advancement has slowed in recent times through promotional disputes and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors ready to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to narrowly win the better rounds.
The crowd's patience diminished during frequent holding, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but several stanzas were hard to judge. However, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to win the European light-heavyweight championship.
The arena became quiet for a extremely emotional homage prior to the bout as a bell ceremony sounded in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised fight night in the location since Hatton's death. As the final chime disappeared, the stillness transitioned to a chant of "there's just one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a significant blow directly to the jaw in the early round, rising shakily to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was damaged soon, heavy swelling forming under his orbital area as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of 114-114, close margin and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero the legend stood for.
