UK light heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close ten-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a contest where the styles never quite clicked, with excessive holding spoiling the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best performance but how do champions respond? They succeed," Buatsi said in the ringside as he was faced disapproval from the spectators.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the victory but the Derbyshire fighter endured the second defeat of his 28-bout career.
The Champion Secures Victory in Slow Encounter
His career development has plateaued in recent years through contract problems and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to edge the more effective periods.
The crowd's patience wore thin during frequent holding, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the co-main event, Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to capture the European light-heavyweight championship.
The venue grew hushed for a deeply poignant tribute prior to the bout as 10 bells tolled in memory of Manchester's own Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the city since Hatton's death. As the final chime disappeared, the silence became a chant of "there's just one Ricky Hatton".
The competition inside the ropes matched the feeling. Arthur was sent to the canvas by a significant blow flush on the face in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up early, noticeable damage forming under his orbital area as the fighters traded hard punches.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor edged the result with judges scorecards of draw, narrow lead and 115-112, but the two athletes were persistent and full of heart, everything their inspiration Hatton stood for.
