British light-heavyweight Buatsi secured a victory yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a match where the techniques never quite clicked, with excessive holding marring the fight.
The officials scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest showing but what do champions do? They succeed," the victor said in the ring as he was greeted with disapproval from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, thirty-one, merited the decision but the British boxer endured the second loss of his 28-bout professional journey.
Buatsi Claims Victory in Slow Encounter
The fighter's advancement has stalled in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a tight guard to barely take the better stanzas.
The spectators' acceptance wore thin during constant clinching, with Parker going down various instances during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a terrific local rivalry battle to capture the European light-heavyweight title.
The stadium fell silent for a deeply poignant tribute prior to the bout as 10 bells sounded in remembrance of the city's native Hatton, who died aged 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the final chime faded, the stillness gave way to a chant of "there's only one Hatton".
The action during the bout equaled the emotion. Arthur was floored by a significant blow flush on the jaw in the initial stage, standing uncertainly to his feet on the count of eight.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was damaged quickly, noticeable damage developing under his face as the fighters landed powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur barely secured the decision with judges scorecards of 114-114, close margin and wider gap, but each competitor were relentless and brave, exactly what their inspiration Hatton stood for.
