British light heavyweight Buatsi secured a victory yet unimpressive debut 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 majority decision in a competitive ten-round fight at the Manchester venue.
But the challenger had his moments, delivering effective clean blows in a fight where the approaches failed to mesh, with excessive holding marring the fight.
Two judges had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," the victor said in the arena as he was met by boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, thirty-one, merited the win but the Derbyshire fighter endured the second defeat of his 28-fight career.
The Champion Claims Victory in Slow Affair
His career development has slowed in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to realize his potential - perhaps in what feels like a ultimate opportunity - under recent support.
He started carefully, with the two boxers willing to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to narrowly win the cleaner stanzas.
The spectators' acceptance decreased during frequent holding, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the supporting bout, Arthur overcame Bradley Rea by close verdict in a excellent local rivalry battle to capture the European light-heavyweight belt.
The venue became quiet for a deeply poignant memorial prior to the bout as ten bells sounded in remembrance of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound faded, the stillness became a chant of "there's just one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the initial stage, standing uncertainly to his position on the referee's count.
But the traditional red attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was injured early, noticeable damage appearing under his eye as the fighters landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the verdict with official scores of 114-114, close margin and wider gap, but the two athletes were persistent and full of heart, exactly what their hero the legend represented.
