UK light heavyweight Buatsi earned a win yet underwhelming debut to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful individual punches in a match where the techniques never quite clicked, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the ringside as he was faced negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, should have been awarded the victory but the contender suffered the second setback of his 28-bout professional journey.
The Champion Secures Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent times through management issues and long spells of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who operated with a protective stance to barely take the more effective periods.
The crowd's patience wore thin during constant clinching, with Parker going down several times during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to capture the continental belt.
The venue became quiet for a profoundly moving homage before the fight as ten bells sounded in memory of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the urban center since Hatton's death. As the final chime faded, the silence became a roar of "there's just one Hatton".
The action within the ring equaled the emotion. Arthur was knocked down by a left hook flush on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling forming under his eye as the pair landed hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur edged the result with official scores of 114-114, narrow lead and wider gap, but each competitor were persistent and courageous, exactly what their inspiration Hatton represented.
