UK light heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a match where the approaches failed to mesh, with excessive holding marring the action.
The scoring judges had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They win," Buatsi commented in the arena as he was met by disapproval from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Various analysts believed the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter experienced the second setback of his 28-fight professional journey.
Buatsi Earns Win in Dull Encounter
His career development has slowed in recent years through management issues and extended periods of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with the two boxers willing to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner stanzas.
The spectators' acceptance diminished during regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the European light-heavyweight belt.
The venue grew hushed for a profoundly moving homage prior to the bout as 10 bells tolled in remembrance of local hero Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound faded, the stillness became a roar of "only one Hatton".
The fighting during the bout equaled the emotion. Arthur was sent to the canvas by a left hook squarely on the face in the second round, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was injured quickly, noticeable damage developing under his face as the duo exchanged hard punches.
Rea, 27, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the action seldom decreased over the full distance.
Arthur narrowly won the decision with official scores of even score, narrow lead and 115-112, but each competitor were determined and brave, all the qualities their hero Hatton embodied.
