UK light heavyweight Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a fight where the styles failed to mesh, with constant grabbing disrupting the action.
Two judges judged it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi stated in the ringside as he was met by disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt Parker, thirty-one, merited the win but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Earns Victory in Dull Match
The fighter's advancement has slowed in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - possibly in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter willing to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who operated with a protective stance to edge the cleaner stanzas.
The crowd's patience decreased amid constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Homage Fuels Local Rivalry Contest
In the co-main event, Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester fight to win the continental belt.
The arena fell silent for a profoundly moving tribute ahead of the contest as ten bells rang in memory of the city's native Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the urban center since Hatton's death. As the final chime faded, the quiet transitioned to a chant of "only one Hatton".
The competition during the bout matched the feeling. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the red-colored equipment of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was damaged early, noticeable damage forming under his face as the duo landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over 12 rounds.
Arthur edged the verdict with official scores of draw, narrow lead and clearer margin, but the two athletes were relentless and full of heart, everything their hero the legend represented.
