UK light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round fight at the Manchester venue.
But the challenger found success, delivering effective single shots in a match where the approaches never quite clicked, with constant grabbing marring the action.
Two judges had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," the victor said in the ringside as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the decision but the Derbyshire fighter suffered the second defeat of his 28-fight career.
The Champion Earns Triumph in Slow Encounter
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the better periods.
The spectators' acceptance wore thin because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a excellent all-Manchester fight to secure the European light heavyweight championship.
The arena became quiet for a extremely emotional tribute prior to the bout as ten bells tolled in memory of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the final chime faded, the silence transitioned to a shout of "there's only one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a left hook directly to the chin in the early round, getting up unsteadily to his feet on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling forming under his orbital area as the duo traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the result with official scores of draw, narrow lead and clearer margin, but both fighters were determined and courageous, all the qualities their idol Hatton represented.
