British light-heavyweight Buatsi secured a victory yet underwhelming start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close ten-round contest at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a match where the approaches failed to mesh, with constant grabbing disrupting the action.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest showing but how do champions respond? They prevail," the victor said in the arena as he was faced disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Earns Victory in Uneventful Affair
The fighter's advancement has slowed in recent times through management issues and long spells of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to overcommit in a cagey opening few 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 light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who operated with a protective stance to narrowly win the better periods.
The spectators' acceptance wore thin amid frequent holding, with the challenger falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by close verdict in a excellent local rivalry fight to win the European light-heavyweight championship.
The stadium became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound faded, the quiet became a shout of "there's just one Hatton".
The action inside the ropes reflected the feeling. Arthur was sent to the canvas by a powerful punch squarely on the face in the second round, getting up unsteadily to his footing on the count of eight.
But the red-colored shorts of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was damaged early, noticeable damage forming under his face as the fighters landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the verdict with scoring tallies of 114-114, close margin and 115-112, but the two athletes were persistent and full of heart, everything their hero Hatton represented.
