British light-heavyweight Buatsi made a winning yet underwhelming start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling single shots in a match where the styles failed to mesh, with excessive holding disrupting the contest.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but what do champions do? They succeed," the victor commented in the ringside as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts felt the challenger, 31, deserved the victory but the Derbyshire fighter experienced the second setback of his 28-match professional journey.
Buatsi Secures Win in Slow Encounter
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to edge the cleaner periods.
The audience's tolerance diminished because of frequent holding, with Parker going down multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a excellent all-Manchester fight to capture the continental title.
The arena became quiet for a extremely emotional tribute before the fight as ten bells rang in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the city since the passing of Hatton. As the concluding sound ended, the stillness gave way to a shout of "only one Hatton".
The action during the bout matched the feeling. The eventual winner was floored by a left hook flush on the jaw in the initial stage, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was injured early, significant bruising developing under his eye as the pair landed powerful shots.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the verdict with judges scorecards of draw, close margin and 115-112, but each competitor were determined and full of heart, exactly what their inspiration the legend represented.
