UK light-heavyweight Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the styles never quite clicked, with excessive holding disrupting the action.
The scoring judges judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," the victor commented in the ring as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, thirty-one, merited the decision but the Derbyshire fighter experienced the second defeat of his 28-fight career.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second bout at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the better stanzas.
The crowd's patience diminished amid regular grabbing, with Parker slipping to the canvas multiple occasions during clinches.
Parker closed well, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by split-decision in a terrific all-Manchester contest to secure the European light heavyweight belt.
The arena became quiet for a deeply poignant memorial before the fight as ten bells rang in memory of local hero Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the concluding sound faded, the stillness became a shout of "there's just one Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was floored by a significant blow flush on the face in the early round, getting up unsteadily to his position on the referee's count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was injured soon, noticeable damage developing under his orbital area as the duo landed powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor narrowly won the decision with official scores of even score, 115-113 and 115-112, but the two athletes were persistent and courageous, exactly what their idol Hatton embodied.
