British light-heavyweight Buatsi earned a win yet disappointing debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger found success, delivering effective single shots in a match where the styles never quite clicked, with constant grabbing marring the contest.
Two judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best display but what defines champions? They prevail," Buatsi commented in the ringside as he was faced negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-fight career.
Buatsi Claims Triumph in Slow Match
His career development has stalled in recent times through contract problems and long spells of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who utilized a tight guard to barely take the more effective stanzas.
The spectators' acceptance decreased amid constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. However, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester battle to win the European light heavyweight belt.
The venue fell silent for a profoundly moving tribute before the fight as 10 bells sounded in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell ended, the quiet became a shout of "there's just one Ricky Hatton".
The competition during the bout matched the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured soon, noticeable damage developing under his orbital area as the duo landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor barely secured the decision with official scores of 114-114, close margin and 115-112, but each competitor were determined and courageous, exactly what their hero Hatton represented.
