British light-heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight 10-round bout at the boxing arena.
But Parker found success, delivering effective clean blows in a contest where the techniques didn't complement each other, with frequent clinching marring the contest.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," the victor stated in the ringside as he was faced disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts believed Parker, 31, deserved the victory but the British boxer endured the second defeat of his 28-match career.
Buatsi Secures Triumph in Slow Encounter
His career development has slowed in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the more effective rounds.
The audience's tolerance diminished during regular grabbing, with the challenger going down various instances during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific local rivalry fight to capture the European light-heavyweight belt.
The venue became quiet for a deeply poignant memorial ahead of the contest as 10 bells rang in remembrance of the city's native Hatton, who died aged 46 in September.
It was the first major televised boxing event in the location since Hatton's passing. As the concluding sound disappeared, the silence became a shout of "only one Hatton".
The action inside the ropes equaled the emotion. Arthur was sent to the canvas by a powerful punch squarely on the face in the initial stage, rising shakily to his feet on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged soon, heavy swelling developing under his eye as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor edged the result with judges scorecards of even score, 115-113 and 115-112, but each competitor were persistent and courageous, everything their hero the legend embodied.
