UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the approaches didn't complement each other, with constant grabbing disrupting the fight.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," the victor said in the arena as he was faced boos from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the win but the British boxer endured the second defeat of his 28-match professional journey.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has slowed in recent times through contract problems and extended periods of lack of fights.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience diminished because of constant clinching, with the challenger falling to the floor several times during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific local rivalry contest to secure the European light heavyweight belt.
The arena fell silent for a extremely emotional tribute before the fight as 10 bells tolled in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the location since Hatton's passing. As the final chime faded, the silence gave way to a shout of "there's only one Hatton".
The competition during the bout matched the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the face in the second round, rising shakily to his footing on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured soon, noticeable damage forming under his orbital area as the pair exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over twelve rounds.
The victor barely secured the result with official scores of even score, narrow lead and wider gap, but the two athletes were determined and brave, all the qualities their idol the legend stood for.
