UK light-heavyweight Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a contest where the techniques never quite clicked, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," the victor said in the ring as he was faced boos from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt the challenger, 31, merited the victory but the contender endured the second setback of his 28-bout professional journey.
Buatsi Earns Triumph in Slow Affair
Buatsi's progress has plateaued in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers ready to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to barely take the better periods.
The spectators' acceptance wore thin during constant clinching, with Parker slipping to the canvas several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Battle
In the co-main event, Arthur defeated Rea by close verdict in a excellent local rivalry contest to secure the continental belt.
The arena grew hushed for a deeply poignant homage before the fight as ten bells rang in honor of local hero Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound disappeared, the stillness became a shout of "there's just one Hatton".
The action during the bout equaled the feeling. Arthur was knocked down by a left hook flush on the chin in the second round, standing uncertainly to his position on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was damaged soon, noticeable damage appearing under his eye 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 seldom decreased over twelve rounds.
The victor edged the result with judges scorecards of even score, close margin and 115-112, but both fighters were persistent and full of heart, everything their hero the legend embodied.
