British light heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches never quite clicked, with frequent clinching marring the contest.
Two judges judged it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor said in the ringside as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Some ringside observers felt the challenger, thirty-one, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Encounter
The fighter's advancement 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 is positioned trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the more effective stanzas.
The audience's tolerance wore thin during frequent holding, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight belt.
The stadium fell silent for a extremely emotional homage prior to the bout as ten bells sounded in honor of the city's native Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the last bell ended, the silence became a chant of "only one Ricky Hatton".
The competition within the ring equaled the sentiment. Arthur was knocked down by a significant blow squarely on the face in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged soon, heavy swelling forming under his face as the pair landed hard punches.
Rea, 27, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the action rarely dropped over the full distance.
Arthur edged the result with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and full of heart, exactly what their idol the legend embodied.
