UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive 10-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles failed to mesh, with frequent clinching disrupting the contest.
The officials judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but how do champions respond? They succeed," the victor stated in the ringside as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought Parker, 31, merited the win but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Match
His career development has plateaued in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a tentative opening few rounds.
Parker, 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 bout at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a solid defense to barely take the better stanzas.
The crowd's patience decreased during regular grabbing, with the challenger going down various instances during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Inspires Manchester Derby Contest
In the supporting bout, Arthur beat Rea by divided decision in a terrific local rivalry battle to secure the European light heavyweight championship.
The venue grew hushed for a deeply poignant tribute prior to the bout as ten bells rang in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the final chime faded, the quiet transitioned to a roar of "there's only one Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a left hook flush on the face in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was marked up quickly, significant bruising forming under his orbital area as the fighters traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were determined and full of heart, all the qualities their inspiration Hatton embodied.
