UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a fight where the styles didn't complement each other, with frequent clinching spoiling the fight.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They win," the victor said in the ring as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, 31, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - maybe in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to narrowly win the better stanzas.
The crowd's patience wore thin amid constant clinching, with Parker falling to the floor several times during clinches.
Parker closed well, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to capture the European light-heavyweight title.
The arena became quiet for a extremely emotional tribute before the fight as ten bells rang in remembrance of local hero Ricky Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime ended, the quiet gave way to a chant of "there's just one Hatton".
The competition within the ring reflected the feeling. The eventual winner was knocked down by a left hook flush on the face in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up quickly, noticeable damage developing under his eye as the duo landed punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but both fighters were persistent and full of heart, all the qualities their inspiration the legend stood for.
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