UK light-heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a match where the approaches never quite clicked, with constant grabbing disrupting the fight.
Two judges judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi commented in the arena as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought the challenger, 31, merited the win but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Secures Win in Uneventful Affair
His career development has plateaued in the past few years through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under new promotional backing.
He began cautiously, with the two boxers willing to take risks in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who operated with a solid defense to barely take the better rounds.
The crowd's patience decreased because of constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester fight to secure the European light-heavyweight belt.
The venue grew hushed for a profoundly moving homage prior to the bout as a bell ceremony rang in remembrance of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime faded, the stillness became a roar of "there's only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was sent to the canvas by a significant blow flush on the jaw in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was marked up quickly, heavy swelling developing under his orbital area as the fighters exchanged hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the pace seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of draw, close margin and clearer margin, but each competitor were persistent and full of heart, exactly what their inspiration the legend embodied.
