British light heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a contest where the styles never quite clicked, with constant grabbing spoiling the fight.
The scoring judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best performance but what defines champions? They succeed," Buatsi said in the ring as he was met by negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts thought Parker, thirty-one, merited the victory but the British boxer endured the second setback of his 28-fight professional journey.
Buatsi Secures Triumph in Dull Encounter
His career development has slowed in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a cagey 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 mid-fight, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to edge the more effective stanzas.
The audience's tolerance wore thin amid constant clinching, with the challenger falling to the floor several times during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. However, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Arthur defeated Bradley Rea by split-decision in a outstanding all-Manchester fight to capture the European light-heavyweight championship.
The arena fell silent for a deeply poignant tribute prior to the bout as 10 bells sounded in remembrance of local hero Ricky Hatton, who died at age 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the last bell faded, the silence transitioned to a chant of "there's just one Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the early round, rising shakily to his position on the eight count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up soon, significant bruising appearing under his face as the fighters traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with scoring tallies of draw, 115-113 and wider gap, but the two athletes were persistent and full of heart, exactly what their hero the legend represented.
