UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive 10-round contest at the Manchester venue.
But the challenger had his moments, delivering effective clean blows in a fight where the techniques failed to mesh, with constant grabbing spoiling the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my best display but how do champions respond? They win," Buatsi said in the ring as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers thought Parker, 31, deserved the victory but the British boxer suffered the second loss of his 28-match professional journey.
Buatsi Secures Victory in Slow Encounter
The fighter's advancement has stalled in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with the two boxers prepared to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the cleaner stanzas.
The spectators' acceptance wore thin amid frequent holding, with Parker falling to the floor various instances during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester battle to win the European light-heavyweight title.
The venue became quiet for a deeply poignant memorial ahead of the contest as ten bells sounded in memory of local hero Hatton, who died 46 years old in last month.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound faded, the stillness gave way to a roar of "there's just one Ricky Hatton".
The action within the ring equaled the feeling. Arthur was floored by a left hook flush on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was marked up soon, significant bruising appearing under his face as the fighters exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor edged the result with official scores of even score, narrow lead and 115-112, but both fighters were relentless and full of heart, all the qualities their idol Hatton embodied.
