UK light-heavyweight Buatsi made a winning yet unimpressive debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close 10-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling clean blows in a match where the techniques didn't complement each other, with excessive holding marring the action.
The scoring judges judged it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best display but what do champions do? They win," Buatsi stated in the arena as he was met by disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought Parker, 31, merited the win but the British boxer suffered the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Affair
The fighter's advancement has slowed in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what feels like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a tentative opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the better periods.
The crowd's patience wore thin amid frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific local rivalry contest to capture the European light heavyweight championship.
The arena grew hushed for a deeply poignant homage prior to the bout as ten bells sounded in memory of Manchester's own Hatton, who succumbed 46 years old in September.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound disappeared, the silence gave way to a chant of "only one Hatton".
The competition inside the ropes equaled the emotion. Arthur was knocked down by a significant blow directly to the chin in the early round, rising shakily to his feet on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged soon, heavy swelling forming under his orbital area as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over 12 rounds.
Arthur barely secured the decision with scoring tallies of draw, narrow lead and wider gap, but each competitor were determined and full of heart, everything their hero the legend embodied.
