UK light-heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a match where the techniques never quite clicked, with excessive holding marring the fight.
Two judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my finest display but what do champions do? They succeed," Buatsi said in the arena as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts felt the challenger, 31, merited the decision but the Derbyshire fighter experienced the second defeat of his 28-bout career.
The Champion Claims Triumph in Slow Affair
His career development has slowed in recent years through promotional disputes and long spells of lack of fights.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to take risks in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who utilized a solid defense to edge the better rounds.
The audience's tolerance decreased amid constant clinching, with Parker going down various instances during clinches.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Homage Fuels Manchester Derby Battle
In the supporting bout, Arthur defeated Rea by split-decision in a terrific all-Manchester contest to capture the European light-heavyweight belt.
The venue fell silent for a extremely emotional homage prior to the bout as ten bells rang in memory of the city's native Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the final chime faded, the silence gave way to a roar of "there's only one Hatton".
The competition during the bout equaled the sentiment. The eventual winner was floored by a powerful punch squarely on the jaw in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was marked up quickly, heavy swelling forming under his eye as the pair exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the result with scoring tallies of draw, narrow lead and wider gap, but both fighters were persistent and full of heart, all the qualities their idol Hatton stood for.
