British light heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a close ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques failed to mesh, with frequent clinching marring the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They succeed," Buatsi commented in the arena as he was greeted with boos from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts thought Parker, 31, merited the decision but the British boxer experienced the second setback of his 28-match professional journey.
Buatsi Claims Win in Dull Encounter
Buatsi's progress has plateaued in the past few years through management issues and considerable time of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to realize his potential - perhaps in what feels like a last chance - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose single earlier setback 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 the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to edge the better rounds.
The crowd's patience diminished during constant clinching, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent local rivalry fight to win the European light-heavyweight title.
The stadium became quiet for a deeply poignant homage before the fight as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the last bell ended, the quiet gave way to a chant of "only one Hatton".
The action inside the ropes reflected the feeling. Arthur was floored by a powerful punch flush on the face in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling developing under his eye as the fighters traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor barely secured the result with scoring tallies of 114-114, close margin and clearer margin, but each competitor were relentless and full of heart, all the qualities their idol the legend represented.
