UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a contest where the approaches never quite clicked, with frequent clinching disrupting the contest.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They succeed," the victor stated in the arena as he was greeted with boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought Parker, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-fight career.
Buatsi Secures Win in Slow Encounter
His career development has slowed in the past few years through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter willing to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the more effective stanzas.
The spectators' acceptance wore thin during constant clinching, with Parker going down various instances during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light heavyweight belt.
The stadium became quiet for a extremely emotional memorial prior to the bout as 10 bells tolled in memory of Manchester's own Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the city since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The action inside the ropes matched the feeling. The eventual winner was knocked down by a significant blow squarely on the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was marked up early, significant bruising appearing under his face as the duo landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the verdict with official scores of draw, narrow lead and wider gap, but the two athletes were persistent and courageous, all the qualities their idol the legend represented.
