British light heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a close 10-round contest at the Manchester venue.
But Parker found success, delivering effective single shots in a match where the techniques never quite clicked, with constant grabbing spoiling the contest.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but what do champions do? They win," Buatsi commented in the arena as he was faced boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the win but the Derbyshire fighter experienced the second setback of his 28-match career.
Buatsi Secures Win in Uneventful Encounter
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to be aggressive in a calculated early stages.
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 the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who worked behind a solid defense to narrowly win the more effective rounds.
The audience's tolerance decreased during constant clinching, with Parker falling to the floor several times during close exchanges.
Parker closed well, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light heavyweight title.
The stadium grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony sounded in honor of local hero Hatton, who passed away 46 years old in September.
It was the first major televised match card in the city since Hatton's passing. As the concluding sound disappeared, the silence gave way to a chant of "there's only one Hatton".
The competition within the ring equaled the feeling. The eventual winner was knocked down by a significant blow directly to the chin in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was marked up quickly, significant bruising forming under his eye as the pair traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the action hardly diminished over twelve rounds.
The victor barely secured the result with official scores of draw, 115-113 and 115-112, but the two athletes were determined and brave, all the qualities their idol Hatton stood for.
