UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close 10-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques didn't complement each other, with excessive holding disrupting the fight.
The officials scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest display but what do champions do? They win," the victor commented in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts believed Parker, thirty-one, deserved the decision but the contender endured the second defeat of his 28-fight career.
Buatsi Earns Triumph in Slow Match
Buatsi's progress has stalled in recent years through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - possibly in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to narrowly win the better periods.
The audience's tolerance diminished amid frequent holding, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light heavyweight title.
The venue became quiet for a profoundly moving tribute before the fight as 10 bells rang in honor of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised match card in the city since Hatton's death. As the final chime faded, the quiet gave way to a shout of "there's only one Ricky Hatton".
The fighting within the ring matched the feeling. The eventual winner was floored by a powerful punch directly to the chin in the initial stage, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was marked up quickly, noticeable damage developing under his eye as the fighters exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the decision with judges scorecards of 114-114, narrow lead and wider gap, but both fighters were determined and brave, exactly what their inspiration the legend represented.
