British light-heavyweight Buatsi made a winning yet disappointing debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close ten-round contest at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a match where the styles failed to mesh, with excessive holding spoiling the fight.
The officials scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They prevail," the victor said in the ringside as he was met by disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts thought the challenger, 31, merited the win but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
Buatsi Earns Win in Uneventful Match
His career development has slowed in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to take risks in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who operated with a protective stance to narrowly win the better periods.
The crowd's patience decreased because of frequent holding, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to secure the European light-heavyweight championship.
The venue grew hushed for a deeply poignant tribute before the fight as 10 bells rang in remembrance of Manchester's own Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime faded, the stillness transitioned to a roar of "there's only one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was floored by a left hook flush on the chin in the early round, rising shakily to his footing on the eight count.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, significant bruising forming under his face as the fighters exchanged punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur edged the verdict with scoring tallies of even score, 115-113 and wider gap, but the two athletes were determined and full of heart, all the qualities their inspiration Hatton represented.
