British light heavyweight Buatsi earned a win yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the styles failed to mesh, with excessive holding marring the fight.
The scoring judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They win," the victor stated in the ring as he was faced negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts believed the challenger, thirty-one, merited the win but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Secures Win in Uneventful Encounter
His career development has plateaued in recent times through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a tight guard to edge the better periods.
The audience's tolerance decreased during constant clinching, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the supporting bout, Arthur beat Bradley Rea by divided decision in a terrific local rivalry battle to secure the continental title.
The venue grew hushed for a profoundly moving memorial ahead of the contest as ten bells sounded in memory of local hero Hatton, who died at age 46 in recently.
It was the premier televised fight night in the urban center since Hatton's death. As the concluding sound faded, the quiet became a shout of "only one Hatton".
The action within the ring matched the feeling. Arthur was knocked down by a left hook directly to the face in the early round, rising shakily to his feet on the eight count.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was marked up soon, heavy swelling forming under his face as the duo traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur barely secured the result with scoring tallies of draw, 115-113 and wider gap, but both fighters were relentless and full of heart, all the qualities their inspiration the legend embodied.
