UK light heavyweight Buatsi secured a victory yet underwhelming start 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 boxing arena.
But Parker had his moments, connecting with powerful individual punches in a match where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi said in the ringside as he was met by boos from the audience.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, 31, should have been awarded the win but the Derbyshire fighter experienced the second defeat of his 28-bout career.
The Champion Earns Win in Uneventful Affair
The fighter's advancement has slowed 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 ultimate opportunity - under recent support.
He started carefully, with both competitors prepared to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was participating in his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to barely take the better stanzas.
The spectators' acceptance diminished amid regular grabbing, with Parker falling to the floor multiple occasions during grappling.
The underdog closed well, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester battle to capture the European light heavyweight championship.
The venue became quiet for a extremely emotional tribute before the fight as 10 bells rang in remembrance of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised fight night in the location since Hatton's death. As the last bell disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The competition inside the ropes matched the feeling. Arthur was knocked down by a significant blow flush on the chin in the early round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured quickly, heavy swelling developing under his orbital area as the pair traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
The victor barely secured the verdict with judges scorecards of even score, close margin and clearer margin, but both fighters were determined and courageous, exactly what their inspiration Hatton represented.
