UK light heavyweight Buatsi secured a victory yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive 10-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a contest where the approaches didn't complement each other, with constant grabbing disrupting the fight.
Two judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi stated in the ring as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Several boxing experts felt the challenger, thirty-one, merited the victory but the British boxer suffered the second setback of his 28-match professional journey.
Buatsi Claims Triumph in Dull Encounter
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the better periods.
The spectators' acceptance diminished because of constant clinching, with the challenger slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to secure the European light heavyweight championship.
The venue fell silent for a profoundly moving tribute prior to the bout as 10 bells tolled in memory of the city's native Hatton, who passed away 46 years old in September.
It was the first major televised match card in the location since Hatton's passing. As the last bell faded, the silence became a roar of "there's just one Hatton".
The competition inside the ropes equaled the emotion. Arthur was sent to the canvas by a significant blow directly to the chin in the second round, rising shakily to his position on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was injured quickly, noticeable damage forming under his orbital area as the pair traded hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur barely secured the verdict with official scores of even score, narrow lead and clearer margin, but each competitor were determined and full of heart, all the qualities their inspiration the legend represented.
