British light heavyweight Buatsi earned a win yet underwhelming start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger found success, delivering effective individual punches in a match where the styles failed to mesh, with constant grabbing spoiling the contest.
The officials had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," the victor stated in the ring as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the victory but the contender experienced the second loss of his 28-bout career.
Buatsi Claims Triumph in Dull Affair
The fighter's advancement has slowed in the past few years through contract problems and long spells of inactivity.
Previously predicted as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under recent support.
He started carefully, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who utilized a protective stance to barely take the cleaner rounds.
The crowd's patience decreased amid frequent holding, with Parker going down multiple occasions during clinches.
Parker ended stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a excellent all-Manchester fight to win the European light heavyweight title.
The stadium grew hushed for a deeply poignant tribute prior to the bout as ten bells tolled in honor of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised match card in the location since the passing of Hatton. As the last bell disappeared, the stillness gave way to a chant of "there's only one Hatton".
The competition during the bout equaled the sentiment. Arthur was knocked down by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, significant bruising forming under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the action rarely dropped over twelve rounds.
The victor edged the decision with scoring tallies of even score, narrow lead and 115-112, but both fighters were relentless and courageous, exactly what their inspiration the legend stood for.
