UK light heavyweight Buatsi made a winning yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight 10-round bout at the boxing arena.
But the challenger found success, landing some telling clean blows in a contest where the approaches never quite clicked, with constant grabbing spoiling the action.
The officials scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They win," the victor stated in the arena as he was met by boos from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the decision but the contender endured the second setback of his 28-match career.
Buatsi Claims Victory in Uneventful Match
His career development has stalled in the past few years through management issues and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He started carefully, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who utilized a solid defense to narrowly win the better stanzas.
The crowd's patience wore thin during regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the co-main event, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to win the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving homage before the fight as 10 bells rang in honor of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound ended, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The competition during the bout reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the jaw in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, significant bruising forming under his face as the duo landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor edged the result with judges scorecards of draw, close margin and 115-112, but the two athletes were persistent and courageous, everything their inspiration the legend embodied.
