UK light heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive 10-round contest at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques failed to mesh, with excessive holding marring the action.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ring as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the decision but the contender suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Triumph in Slow Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to edge the more effective stanzas.
The audience's tolerance decreased because of regular grabbing, with Parker going down several times during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Arthur overcame Bradley Rea by close verdict in a terrific local rivalry fight to secure the continental belt.
The stadium became quiet for a deeply poignant memorial before the fight as ten bells sounded in honor of local hero Ricky Hatton, who died 46 years old in September.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime faded, the quiet became a shout of "there's just one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the early round, standing uncertainly to his feet on the eight count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged early, significant bruising developing under his eye as the duo exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor barely secured the decision with judges scorecards of 114-114, close margin and wider gap, but each competitor were relentless and courageous, all the qualities their idol Hatton represented.
