UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the approaches failed to mesh, with constant grabbing spoiling the fight.
The officials judged it 96-94 to Buatsi, with the final judge 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 disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th pro success.
Various analysts felt the challenger, 31, deserved the win but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
Buatsi Earns Win in Dull Affair
The fighter's advancement has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He started carefully, with both competitors ready to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The spectators' acceptance wore thin during constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Homage Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the continental belt.
The arena grew hushed for a deeply poignant homage prior to the bout as 10 bells sounded in remembrance of local hero Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the city since Hatton's death. As the final chime disappeared, the quiet became a shout of "there's just one Hatton".
The fighting during the bout matched the feeling. The eventual winner was floored by a significant blow flush on the face in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was marked up early, noticeable damage developing under his face as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur edged the decision with scoring tallies of draw, close margin and wider gap, but both fighters were persistent and brave, everything their hero the legend embodied.
