British light-heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But Parker found success, delivering effective single shots in a fight where the approaches didn't complement each other, with constant grabbing spoiling the action.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor said in the arena as he was met by boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Various analysts felt Parker, 31, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
Buatsi Secures Win in Dull Encounter
His career development 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 finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He began cautiously, with both competitors prepared to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to edge the better rounds.
The audience's tolerance decreased amid constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
Parker finished the stronger, but many rounds were difficult to score. However, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Fuels Manchester Derby Battle
In the co-main event, Arthur overcame Rea by divided decision in a excellent all-Manchester battle to capture the European light heavyweight title.
The arena became quiet for a profoundly moving memorial prior to the bout as 10 bells rang in honor of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised match card in the urban center since Hatton's death. As the final chime ended, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, heavy swelling forming under his face as the duo landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the decision with judges scorecards of draw, 115-113 and clearer margin, but the two athletes were persistent and courageous, all the qualities their hero Hatton represented.
