UK light heavyweight Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a fight where the styles never quite clicked, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They prevail," the victor said in the ring as he was greeted with negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Various analysts thought the challenger, 31, deserved the victory but the British boxer suffered the second defeat of his 28-bout career.
Buatsi Earns Triumph in Dull Affair
His career development has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with both competitors ready to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who utilized a tight guard to edge the better periods.
The spectators' acceptance diminished amid constant clinching, with Parker falling to the floor several times during close exchanges.
Parker closed well, but many rounds were challenging to assess. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent all-Manchester contest to secure the continental championship.
The arena became quiet for a extremely emotional tribute before the fight as ten bells rang in memory of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the quiet gave way to a shout of "only one Hatton".
The action inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was marked up soon, heavy swelling developing under his eye as the fighters traded powerful shots.
Rea, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of draw, 115-113 and clearer margin, but the two athletes were determined and courageous, all the qualities their inspiration Hatton embodied.
