UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a contest where the techniques didn't complement each other, with constant grabbing disrupting the fight.
The officials scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but what do champions do? They succeed," the victor commented in the ring as he was faced disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the British boxer endured the second defeat of his 28-fight career.
The Champion Claims Triumph in Uneventful Encounter
Buatsi's progress has stalled in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a tight guard to narrowly win the more effective periods.
The audience's tolerance diminished amid regular grabbing, with the challenger slipping to the canvas various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Inspires Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester fight to capture the continental title.
The stadium fell silent for a extremely emotional homage before the fight as ten bells sounded in memory of local hero Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the concluding sound faded, the quiet became a chant of "only one Hatton".
The fighting inside the ropes matched the emotion. Arthur was sent to the canvas by a left hook flush on the jaw in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was injured soon, significant bruising developing under his orbital area as the pair traded hard punches.
The challenger, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the result with official scores of 114-114, 115-113 and 115-112, but each competitor were persistent and brave, everything their inspiration Hatton represented.
