UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a tight ten-round contest at the boxing arena.
But Parker found success, delivering effective single shots in a match where the approaches didn't complement each other, with excessive holding disrupting the contest.
The scoring judges judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They win," Buatsi commented in the ringside as he was faced boos from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Several boxing experts thought Parker, thirty-one, should have been awarded the win but the contender experienced the second defeat of his 28-bout career.
Buatsi Secures Win in Uneventful Affair
His career development has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under recent support.
He began cautiously, with the two boxers prepared to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who utilized a protective stance to edge the better stanzas.
The audience's tolerance diminished because of constant clinching, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester contest to secure the European light-heavyweight title.
The venue fell silent for a deeply poignant homage ahead of the contest as 10 bells tolled in memory of Manchester's own Hatton, who died aged 46 in last month.
It was the first major televised match card in the location since the passing of Hatton. As the final chime disappeared, the quiet became a roar of "only one Hatton".
The action inside the ropes matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the early round, rising shakily to his feet on the count of eight.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was marked up early, heavy swelling developing under his orbital area as the duo landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of even score, close margin and clearer margin, but both fighters were determined and courageous, everything their inspiration Hatton represented.
