British light-heavyweight Buatsi earned a win yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a match where the styles never quite clicked, with constant grabbing marring the action.
Two judges judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," the victor said in the ring as he was faced negative reactions 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 pro success.
Several boxing experts felt the challenger, thirty-one, merited the decision but the Derbyshire fighter endured the second loss of his 28-fight professional journey.
Buatsi Secures Victory in Slow Encounter
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to edge the more effective stanzas.
The spectators' acceptance decreased during frequent holding, with Parker going down multiple occasions during grappling.
Parker ended stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Homage Inspires All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester fight to secure the continental championship.
The venue fell silent for a profoundly moving memorial before the fight as a bell ceremony rang in remembrance of Manchester's own Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the last bell faded, the quiet became a shout of "there's only one Hatton".
The competition within the ring equaled the emotion. The eventual winner was knocked down by a powerful punch squarely on the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was injured soon, significant bruising appearing under his face as the fighters landed punishing blows.
The challenger, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of even score, 115-113 and wider gap, but the two athletes were determined and brave, everything their inspiration the legend represented.
