UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a close 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the approaches didn't complement each other, with excessive holding marring the action.
The officials judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best showing but what do champions do? They prevail," the victor commented in the ringside as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers felt Parker, thirty-one, should have been awarded the decision but the contender endured the second setback of his 28-bout career.
The Champion Earns Victory in Dull Match
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to barely take the better rounds.
The spectators' acceptance diminished because of regular grabbing, with the challenger going down various instances during grappling.
Parker closed well, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester contest to win the European light heavyweight belt.
The arena fell silent for a extremely emotional memorial prior to the bout as a bell ceremony rang in honor of Manchester's own Hatton, who died at age 46 in September.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound faded, the stillness became a roar of "there's only one Hatton".
The action within the ring matched the feeling. Arthur was knocked down by a significant blow directly to the face in the second round, rising shakily to his position on the count of eight.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured soon, heavy swelling forming under his orbital area as the pair exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur barely secured the result with official scores of even score, 115-113 and clearer margin, but both fighters were relentless and courageous, everything their idol the legend embodied.
