British light heavyweight Buatsi earned a win yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a contest where the approaches failed to mesh, with frequent clinching marring the contest.
The officials judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi said in the ringside as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Some ringside observers believed Parker, 31, should have been awarded the win but the British boxer endured the second defeat of his 28-match professional journey.
The Champion Earns Victory in Uneventful Affair
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers willing to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to barely take the cleaner stanzas.
The audience's tolerance wore thin because of constant clinching, with the challenger slipping to the canvas various instances during grappling.
Parker closed well, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a outstanding local rivalry battle to secure the European light-heavyweight championship.
The stadium fell silent for a extremely emotional homage ahead of the contest as 10 bells sounded in remembrance of the city's native Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the last bell faded, the quiet became a roar of "only one Hatton".
The fighting inside the ropes matched the emotion. Arthur was sent to the canvas by a left hook directly to the face in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was damaged early, heavy swelling forming under his face as the pair exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the verdict with official scores of draw, narrow lead and clearer margin, but both fighters were persistent and courageous, all the qualities their hero Hatton represented.
