UK light heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches failed to mesh, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi said in the arena as he was met by negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
The Champion Claims Win in Slow Encounter
Buatsi's progress has slowed in recent times through management issues and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now stands 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 take risks in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to edge the cleaner periods.
The crowd's patience diminished during regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester battle to secure the European light-heavyweight championship.
The venue became quiet for a profoundly moving memorial prior to the bout as 10 bells sounded in honor of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell faded, the quiet gave way to a chant of "there's just one Hatton".
The action inside the ropes equaled the emotion. Arthur was knocked down by a significant blow directly to the chin in the early round, rising shakily to his footing on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was marked up soon, heavy swelling appearing under his eye as the duo exchanged powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of draw, 115-113 and 115-112, but each competitor were determined and courageous, all the qualities their inspiration the legend embodied.
