British light-heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a fight where the techniques never quite clicked, with constant grabbing disrupting the contest.
The officials judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," Buatsi stated in the ringside as he was met by negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, 31, deserved the win but the British boxer endured the second setback of his 28-bout career.
Buatsi Earns Win in Slow Affair
His career development has slowed in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The spectators' acceptance decreased during frequent holding, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific local rivalry contest to win the European light-heavyweight championship.
The venue fell silent for a deeply poignant memorial ahead of the contest as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime ended, the silence gave way to a shout of "there's just one Ricky Hatton".
The fighting during the bout reflected the feeling. The eventual winner was knocked down by a powerful punch flush on the face in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured quickly, noticeable damage appearing under his eye as the pair landed hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of draw, 115-113 and clearer margin, but each competitor were determined and full of heart, all the qualities their idol Hatton embodied.
