British light-heavyweight Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a fight where the styles didn't complement each other, with frequent clinching spoiling the contest.
The officials had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi stated in the arena as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers felt Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second setback of his 28-fight career.
The Champion Claims Win in Slow Affair
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to overcommit in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to narrowly win the better stanzas.
The crowd's patience decreased because of regular grabbing, with the challenger going down various instances during grappling.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific all-Manchester fight to capture the European light-heavyweight championship.
The arena became quiet for a deeply poignant homage before the fight as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound ended, the stillness gave way to a roar of "there's only one Hatton".
The fighting within the ring reflected the emotion. Arthur was knocked down by a left hook flush on the jaw in the second round, getting up unsteadily to his feet on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was damaged quickly, noticeable damage developing under his face as the fighters traded hard punches.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur narrowly won the result with official scores of draw, close margin and wider gap, but each competitor were persistent and courageous, all the qualities their idol Hatton represented.
