UK light heavyweight Buatsi earned a win yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a fight where the approaches failed to mesh, with constant grabbing spoiling the action.
The scoring judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but what do champions do? They prevail," Buatsi said in the arena as he was faced boos from the crowd.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers felt Parker, 31, merited the win but the Derbyshire fighter suffered the second setback of his 28-fight career.
The Champion Earns Victory in Dull Affair
Buatsi's progress has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - possibly in what feels like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a cagey opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to barely take the better rounds.
The audience's tolerance decreased during frequent holding, with Parker falling to the floor several times during grappling.
The underdog closed well, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the co-main event, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight belt.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as 10 bells rang in remembrance of local hero Ricky Hatton, who died aged 46 in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the last bell faded, the silence gave way to a roar of "only one Hatton".
The competition within the ring reflected the sentiment. Arthur was sent to the canvas by a left hook directly to the face in the second round, getting up unsteadily to his feet on the referee's count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising developing under his eye as the pair exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the result with scoring tallies of draw, close margin and wider gap, but the two athletes were persistent and courageous, all the qualities their hero the legend represented.
