British light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They win," Buatsi said in the ringside as he was met by negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the win but the British boxer experienced the second setback of his 28-bout career.
Buatsi Earns Triumph in Slow Match
His career development has stalled in recent years through promotional disputes and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a last chance - under fresh management.
He started carefully, with the two boxers ready to be aggressive in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who operated with a solid defense to edge the more effective stanzas.
The audience's tolerance diminished amid constant clinching, with the challenger going down several times during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific local rivalry fight to secure the European light-heavyweight title.
The stadium became quiet for a deeply poignant homage prior to the bout as 10 bells sounded in honor of local hero Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound ended, the silence became a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a significant blow directly to the chin in the early round, rising shakily to his feet on the referee's count.
But the red-colored equipment of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was marked up early, heavy swelling forming under his face as the duo exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
Arthur narrowly won the result with scoring tallies of draw, close margin and clearer margin, but the two athletes were persistent and courageous, exactly what their hero the legend stood for.
