UK light heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a match where the techniques didn't complement each other, with excessive holding spoiling the contest.
Two judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They win," the victor said in the ringside as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Various analysts believed the challenger, thirty-one, merited the win but the contender experienced the second loss of his 28-fight career.
The Champion Earns Triumph in Slow Affair
His career development has slowed in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to realize his potential - maybe in what appears to be a last chance - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who utilized a tight guard to narrowly win the cleaner stanzas.
The audience's tolerance decreased during regular grabbing, with the challenger falling to the floor several times during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to capture the European light heavyweight belt.
The stadium grew hushed for a profoundly moving homage prior to the bout as ten bells tolled in honor of the city's native Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell ended, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring matched the feeling. Arthur was sent to the canvas by a left hook flush on the face in the early round, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling developing under his eye as the pair traded hard punches.
Rea, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
The victor edged the result with scoring tallies of even score, close margin and wider gap, but both fighters were determined and full of heart, everything their idol Hatton represented.
