British light heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a match where the styles didn't complement each other, with constant grabbing marring the contest.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi stated in the ringside as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, 31, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Slow Affair
His career development has plateaued in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who worked behind a tight guard to edge the more effective rounds.
The spectators' acceptance decreased amid constant clinching, with Parker falling to the floor various instances during close exchanges.
Parker closed well, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester contest to capture the continental championship.
The venue fell silent for a profoundly moving homage ahead of the contest as a bell ceremony rang in memory of local hero Hatton, who passed away aged 46 in September.
It was the first major televised boxing event in the location since Hatton's death. As the last bell ended, the stillness became a chant of "only one Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was floored by a left hook squarely on the face in the early round, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage appearing under his eye as the pair traded powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur narrowly won the result with official scores of draw, close margin and clearer margin, but the two athletes were relentless and brave, exactly what their hero the legend embodied.
