British 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 punch output appeared to earn him a split decision in a competitive 10-round contest at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a contest where the techniques never quite clicked, with excessive holding spoiling the action.
The officials scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They win," the victor commented in the ringside as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Various analysts felt the challenger, thirty-one, deserved the win but the contender experienced the second defeat of his 28-bout career.
Buatsi Claims Triumph in Uneventful Match
The fighter's advancement has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to edge the more effective rounds.
The crowd's patience decreased because of regular grabbing, with Parker falling to the floor several times during clinches.
Parker ended stronger, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to win the European light heavyweight championship.
The arena fell silent for a extremely emotional memorial ahead of the contest as ten bells sounded in honor of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound ended, the stillness gave way to a shout of "only one Ricky Hatton".
The action inside the ropes reflected the emotion. Arthur was knocked down by a significant blow squarely on the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored attire of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was injured quickly, significant bruising forming under his face as the fighters exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur narrowly won the decision with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were persistent and brave, all the qualities their hero Hatton embodied.
