UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger had his moments, delivering effective clean blows in a fight where the styles failed to mesh, with excessive holding disrupting the contest.
The officials scored it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi commented in the arena as he was greeted with disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts felt the challenger, thirty-one, deserved the win but the contender experienced the second defeat of his 28-match career.
Buatsi Earns Win in Slow Affair
Buatsi's progress has slowed in the past few years through contract problems and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to edge the cleaner stanzas.
The crowd's patience diminished because of frequent holding, with Parker going down several times during grappling.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor moves on, but with the presence of hard hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a terrific all-Manchester battle to secure the European light heavyweight championship.
The stadium fell silent for a deeply poignant homage before the fight as a bell ceremony sounded in memory of the city's native Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the city since Hatton's passing. As the last bell ended, the quiet gave way to a chant of "there's just one Ricky Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the face in the second round, rising shakily to his position on the referee's count.
But the traditional red shorts of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was injured early, significant bruising developing under his eye as the duo exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action rarely dropped over the full distance.
Arthur narrowly won the verdict with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were persistent and full of heart, exactly what their idol Hatton represented.
