UK light heavyweight Buatsi made a winning yet unimpressive debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a match where the techniques never quite clicked, with constant grabbing spoiling the contest.
Two judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi commented in the ring as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, 31, merited the victory but the Derbyshire fighter experienced the second defeat of his 28-fight professional journey.
Buatsi Claims Triumph in Dull Match
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to edge the cleaner stanzas.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but several stanzas were hard to judge. Although, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a outstanding local rivalry fight to capture the continental title.
The stadium fell silent for a extremely emotional tribute prior to the bout as ten bells rang in honor of the city's native Hatton, who passed away aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the final chime faded, the stillness became a chant of "there's just one Hatton".
The action within the ring reflected the emotion. Arthur was sent to the canvas by a left hook squarely on the face in the early round, rising shakily to his footing on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, significant bruising developing under his orbital area as the pair landed hard punches.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over 12 rounds.
The victor barely secured the result with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were determined and courageous, exactly what their inspiration Hatton stood for.
