UK light-heavyweight Buatsi secured a victory yet disappointing debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight ten-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a contest where the styles didn't complement each other, with frequent clinching spoiling the contest.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They win," the victor commented in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Various analysts believed the challenger, 31, should have been awarded the victory but the British boxer experienced the second setback of his 28-match career.
Buatsi Secures Triumph in Slow Affair
His career development has plateaued in recent times through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with both competitors willing to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The crowd's patience diminished because of constant clinching, with the challenger going down several times during grappling.
The underdog ended stronger, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Fight
In the secondary fight, Arthur overcame Rea by split-decision in a terrific all-Manchester contest to capture the European light heavyweight championship.
The venue grew hushed for a deeply poignant tribute before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell ended, the stillness became a roar of "only one Hatton".
The action within the ring reflected the feeling. Arthur was knocked down by a significant blow squarely on the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the red-colored equipment of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was injured quickly, heavy swelling forming under his face as the duo landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were determined and brave, exactly what their hero Hatton stood for.
