British light-heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a match where the approaches failed to mesh, with frequent clinching marring the contest.
Two judges had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi said in the arena as he was greeted with negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Several boxing experts felt Parker, 31, merited the decision but the British boxer endured the second defeat of his 28-bout career.
Buatsi Claims Win in Dull Match
Buatsi's progress has stalled in recent times through promotional disputes and considerable time of lack of fights.
Once tipped as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter ready to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a tight guard to barely take the cleaner periods.
The spectators' acceptance decreased because of frequent holding, with Parker falling to the floor several times during close exchanges.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester battle to capture the European light heavyweight title.
The venue fell silent for a deeply poignant memorial ahead of the contest as ten bells sounded in remembrance of local hero Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the last bell faded, the quiet gave way to a shout of "only one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a significant blow flush on the face in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged soon, heavy swelling forming under his eye as the duo exchanged punishing blows.
Rea, 27, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of even score, close margin and 115-112, but both fighters were persistent and full of heart, exactly what their idol Hatton represented.
