British 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 medal winner's punch output appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a match where the approaches never quite clicked, with constant grabbing marring the action.
Two judges scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my best display but what defines champions? They succeed," Buatsi commented in the ring as he was faced negative reactions from the crowd.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, deserved the win but the contender suffered the second loss of his 28-match career.
The Champion Earns Win in Dull Encounter
His career development has plateaued in recent years through contract problems and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who operated with a protective stance to narrowly win the cleaner stanzas.
The spectators' acceptance wore thin because of constant clinching, with Parker going down various instances during close exchanges.
Parker closed well, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester battle to capture the continental title.
The arena grew hushed for a deeply poignant homage prior to the bout as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the location since Hatton's passing. As the concluding sound faded, the silence gave way to a chant of "there's only one Hatton".
The action inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the early round, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was injured soon, heavy swelling developing under his face as the fighters traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with powerful upward punches as the intensity seldom decreased over the full distance.
The victor narrowly won the verdict with official scores of even score, narrow lead and 115-112, but both fighters were persistent and full of heart, everything their idol the legend represented.
