UK light heavyweight Buatsi earned a win yet unimpressive start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight ten-round contest at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the approaches didn't complement each other, with constant grabbing marring the fight.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi stated in the arena as he was met by negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the win but the British boxer suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Dull Match
Buatsi's progress has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter ready to be aggressive in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a protective stance to edge the better stanzas.
The crowd's patience decreased because of constant clinching, with Parker falling to the floor various instances during close exchanges.
The underdog closed well, but many rounds were difficult to score. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry fight to win the continental title.
The arena fell silent for a extremely emotional homage ahead of the contest as ten bells rang in memory of Manchester's own Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime ended, the stillness became a chant of "only one Ricky Hatton".
The fighting inside the ropes matched the feeling. Arthur was floored by a left hook flush on the chin in the initial stage, rising shakily to his feet on the referee's count.
But the traditional red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured early, significant bruising appearing under his face as the pair landed punishing blows.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the decision with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and courageous, exactly what their hero Hatton represented.
