British light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight ten-round fight at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a fight where the techniques didn't complement each other, with constant grabbing marring the contest.
The officials scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They win," the victor stated in the ring as he was faced boos from the crowd.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, merited the win but the contender experienced the second defeat of his 28-fight career.
Buatsi Secures Win in Slow Match
His career development has plateaued in recent years through management issues and considerable time of inactivity.
Previously predicted as the future of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who operated with a protective stance to barely take the cleaner periods.
The audience's tolerance diminished amid constant clinching, with Parker falling to the floor several times during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a outstanding all-Manchester battle to win the European light heavyweight belt.
The venue grew hushed for a extremely emotional homage before the fight as a bell ceremony rang in honor of local hero Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the concluding sound ended, the stillness gave way to a chant of "only one Hatton".
The competition during the bout equaled the feeling. Arthur was floored by a powerful punch flush on the face in the early round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was damaged quickly, heavy swelling developing under his face as the fighters exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over twelve rounds.
The victor edged the decision with scoring tallies of draw, narrow lead and clearer margin, but the two athletes were determined and brave, all the qualities their idol Hatton represented.
