British light heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a contest where the styles never quite clicked, with excessive holding marring the fight.
The officials judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best showing but what do champions do? They succeed," Buatsi said in the ringside as he was faced negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Several boxing experts thought the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
Buatsi Secures Triumph in Uneventful Encounter
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to edge the better rounds.
The audience's tolerance wore thin because of constant clinching, with the challenger going down various instances during grappling.
The underdog finished the stronger, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a excellent all-Manchester contest to capture the continental championship.
The venue grew hushed for a profoundly moving memorial before the fight as 10 bells tolled in honor of Manchester's own Hatton, who succumbed 46 years old in September.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound ended, the stillness became a shout of "only one Ricky Hatton".
The competition inside the ropes matched the emotion. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was damaged early, heavy swelling forming under his face as the duo traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor narrowly won the verdict with scoring tallies of even score, narrow lead and wider gap, but the two athletes were persistent and brave, exactly what their hero Hatton represented.
