UK light-heavyweight Buatsi earned a win yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a match where the approaches didn't complement each other, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They win," the victor said in the ring as he was faced boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the win but the British boxer suffered the second setback of his 28-fight professional journey.
Buatsi Earns Victory in Dull Encounter
Buatsi's progress has plateaued in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He began cautiously, with the two boxers prepared to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a protective stance to narrowly win the better rounds.
The spectators' acceptance decreased during constant clinching, with Parker going down various instances during grappling.
Parker closed well, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry contest to capture the European light-heavyweight belt.
The arena grew hushed for a extremely emotional tribute prior to the bout as ten bells rang in memory of local hero Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the last bell disappeared, the stillness transitioned to a roar of "there's only one Hatton".
The action within the ring matched the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was damaged quickly, noticeable damage appearing under his face as the duo traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the verdict with judges scorecards of even score, 115-113 and 115-112, but each competitor were determined and brave, exactly what their hero Hatton embodied.
