British light heavyweight Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close ten-round contest at the Manchester venue.
But the challenger found success, delivering effective clean blows in a contest where the styles failed to mesh, with excessive holding disrupting the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor said in the arena as he was met by disapproval from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, merited the victory but the British boxer experienced the second loss of his 28-bout career.
Buatsi Earns Win in Uneventful Match
His career development has slowed in recent years through management issues and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who worked behind a solid defense to barely take the more effective rounds.
The crowd's patience diminished because of frequent holding, with Parker going down several times during clinches.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight title.
The arena fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in remembrance of the city's native Hatton, who passed away at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's death. As the last bell faded, the quiet became a shout of "there's just one Hatton".
The competition during the bout matched the emotion. The eventual winner was floored by a left hook directly to the face in the second round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged quickly, noticeable damage forming under his eye as the fighters exchanged punishing blows.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over twelve rounds.
Arthur barely secured the decision with scoring tallies of even score, 115-113 and 115-112, but each competitor were relentless and courageous, all the qualities their idol Hatton represented.
