British light heavyweight Buatsi secured a victory yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a match where the techniques didn't complement each other, with frequent clinching spoiling the contest.
The scoring judges scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my finest performance but how do champions respond? They win," Buatsi said in the ringside as he was faced negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Claims Victory in Dull Affair
The fighter's advancement has slowed in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers willing to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who utilized a tight guard to edge the better rounds.
The audience's tolerance wore thin during regular grabbing, with Parker falling to the floor several times during close exchanges.
The underdog ended stronger, but several stanzas were hard to judge. However, former world champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester contest to win the European light-heavyweight belt.
The arena fell silent for a profoundly moving tribute before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised match card in the urban center since the passing of Hatton. As the concluding sound ended, the quiet became a shout of "there's just one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was floored by a powerful punch flush on the face in the second round, getting up unsteadily to his position on the count of eight.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up quickly, significant bruising forming under his eye as the duo traded punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of even score, narrow lead and wider gap, but each competitor were relentless and courageous, everything their idol Hatton stood for.
