UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a match where the styles failed to mesh, with excessive holding marring the contest.
Two judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi commented in the ringside as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to secure a 20th career victory.
Various analysts felt the challenger, thirty-one, merited the win but the British boxer suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Match
His career development has plateaued in recent times through management issues and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the cleaner periods.
The audience's tolerance diminished because of regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. However, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester battle to secure the European light-heavyweight belt.
The stadium grew hushed for a deeply poignant tribute prior to the bout as 10 bells rang in remembrance of Manchester's own Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the last bell disappeared, the stillness gave way to a roar of "there's just one Hatton".
The fighting inside the ropes reflected the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was damaged quickly, noticeable damage forming under his orbital area as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over the full distance.
The victor edged the decision with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were persistent and brave, exactly what their hero Hatton stood for.
