UK light-heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a match where the styles never quite clicked, with excessive holding marring the action.
The officials scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," Buatsi said in the ring as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, merited the victory but the contender suffered the second loss of his 28-bout professional journey.
The Champion Earns Win in Uneventful Match
His career development has slowed in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with both competitors ready to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to narrowly win the more effective rounds.
The spectators' acceptance decreased during constant clinching, with Parker going down multiple occasions during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry battle to win the European light heavyweight belt.
The arena became quiet for a deeply poignant memorial prior to the bout as a bell ceremony tolled in remembrance of local hero Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a shout of "only one Ricky Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was knocked down by a significant blow flush on the face in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up quickly, heavy swelling appearing under his orbital area as the fighters exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor barely secured the decision with judges scorecards of 114-114, close margin and wider gap, but the two athletes were relentless and full of heart, all the qualities their hero Hatton represented.
