UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round fight at the Manchester venue.
But the challenger found success, delivering effective clean blows in a contest where the techniques didn't complement each other, with excessive holding disrupting the contest.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They succeed," the victor stated in the ring as he was met by boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts thought the challenger, 31, deserved the decision but the British boxer suffered the second setback of his 28-match career.
Buatsi Claims Victory in Dull Match
The fighter's advancement has stalled in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors prepared to be aggressive in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who utilized a tight guard to narrowly win the cleaner stanzas.
The crowd's patience wore thin because of regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester battle to capture the continental title.
The venue became quiet for a profoundly moving memorial ahead of the contest as ten bells tolled in memory of Manchester's own Hatton, who passed away 46 years old in last month.
It was the premier televised boxing event in the city since the passing of Hatton. As the last bell ended, the quiet transitioned to a roar of "only one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the face in the early round, rising shakily to his footing on the eight count.
But the traditional red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged quickly, heavy swelling forming under his eye as the duo landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the verdict with official scores of draw, 115-113 and wider gap, but both fighters were persistent and brave, exactly what their hero Hatton represented.
