British light-heavyweight Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a contest where the techniques didn't complement each other, with excessive holding marring the action.
The officials had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," Buatsi commented in the ringside as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers felt the challenger, 31, merited the victory but the British boxer experienced the second setback of his 28-match professional journey.
The Champion Earns Win in Slow Encounter
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a last chance - under fresh management.
He started carefully, with both competitors willing to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to narrowly win the more effective rounds.
The spectators' acceptance diminished because of frequent holding, with the challenger slipping to the canvas various instances during clinches.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester fight to capture the European light heavyweight championship.
The arena became quiet for a deeply poignant tribute before the fight as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell faded, the silence became a chant of "only one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was sent to the canvas by a left hook directly to the face in the initial stage, standing uncertainly to his footing on the count of eight.
But the red-colored shorts of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged early, significant bruising forming under his eye as the fighters exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the verdict with scoring tallies of even score, close margin and clearer margin, but each competitor were relentless and courageous, exactly what their idol Hatton stood for.
