UK light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a fight where the styles never quite clicked, with frequent clinching marring the fight.
Two judges judged it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," the victor commented in the ring as he was greeted with disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers thought Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
The Champion Secures Win in Uneventful Affair
Buatsi's progress has slowed in the past few years through management issues and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a last chance - under recent support.
He started carefully, with the two boxers prepared to take risks in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better periods.
The audience's tolerance diminished during frequent holding, with the challenger falling to the floor several times during grappling.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry contest to secure the European light heavyweight title.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as a bell ceremony sounded in memory of local hero Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's death. As the last bell ended, the stillness transitioned to a chant of "there's just one Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was floored by a significant blow directly to the chin in the initial stage, rising shakily to his footing on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising developing under his orbital area as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor edged the verdict with official scores of even score, close margin and clearer margin, but each competitor were determined and full of heart, exactly what their idol Hatton stood for.
