British light heavyweight Buatsi made a winning yet underwhelming start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a contest where the styles didn't complement each other, with frequent clinching marring the action.
Two judges scored it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest showing but what do champions do? They prevail," the victor said in the arena as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers believed the challenger, 31, merited the victory but the British boxer suffered the second defeat of his 28-fight professional journey.
The Champion Earns Win in Slow Match
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to barely take the more effective stanzas.
The spectators' acceptance diminished during frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the co-main event, Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester fight to capture the European light heavyweight belt.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound disappeared, the silence transitioned to a shout of "there's only one Ricky Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a significant blow directly to the chin in the second round, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up soon, noticeable damage forming under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the result with scoring tallies of draw, close margin and 115-112, but the two athletes were persistent and courageous, exactly what their hero the legend embodied.
