UK light heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a close ten-round fight at the boxing arena.
But Parker landed significant shots, landing some telling individual punches in a fight where the approaches didn't complement each other, with frequent clinching spoiling the contest.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what defines champions? They prevail," the victor stated in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Several boxing experts believed the challenger, 31, should have been awarded the decision but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Secures Triumph in Uneventful Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a solid defense to narrowly win the better stanzas.
The audience's tolerance diminished amid constant clinching, with Parker going down multiple occasions during grappling.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Arthur beat Rea by close verdict in a outstanding all-Manchester battle to secure the European light heavyweight championship.
The stadium grew hushed for a extremely emotional memorial before the fight as a bell ceremony sounded in memory of the city's native Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the urban center since Hatton's death. As the last bell disappeared, the silence transitioned to a shout of "there's just one Ricky Hatton".
The fighting inside the ropes matched the emotion. The eventual winner was floored by a significant blow flush on the face in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was damaged early, significant bruising developing under his eye as the fighters exchanged hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, close margin and clearer margin, but each competitor were determined and courageous, everything their inspiration the legend represented.
