UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the approaches never quite clicked, with frequent clinching disrupting the contest.
Two judges judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi stated in the ringside as he was greeted with boos from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts believed the challenger, 31, should have been awarded the decision but the British boxer endured the second setback of his 28-match professional journey.
The Champion Claims Victory in Uneventful Encounter
Buatsi's progress has plateaued in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what feels like a ultimate opportunity - under fresh management.
He started carefully, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to barely take the more effective stanzas.
The audience's tolerance diminished during regular grabbing, with Parker going down multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a terrific local rivalry contest to win the European light heavyweight title.
The venue fell silent for a deeply poignant tribute prior to the bout as ten bells tolled in memory of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the final chime ended, the silence transitioned to a shout of "there's just one Hatton".
The action inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a left hook directly to the face in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up soon, noticeable damage forming under his orbital area as the duo traded punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor edged the verdict with official scores of draw, close margin and wider gap, but each competitor were persistent and brave, all the qualities their inspiration Hatton represented.
