British light heavyweight Buatsi secured a victory yet unimpressive debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight ten-round fight at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the styles never quite clicked, with excessive holding spoiling the action.
The officials judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what do champions do? They win," the victor commented in the ringside as he was met by boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers believed the challenger, 31, merited the win but the contender endured the second loss of his 28-bout career.
Buatsi Secures Win in Slow Encounter
The fighter's advancement has stalled in recent times through management issues and considerable time of lack of fights.
Once tipped as the rising talent of the British fight scene, he now stands trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who worked behind a tight guard to narrowly win the better periods.
The crowd's patience decreased amid frequent holding, with Parker falling to the floor several times during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a terrific all-Manchester battle to win the European light-heavyweight belt.
The arena became quiet for a profoundly moving memorial before the fight as ten bells rang in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the city since Hatton's death. As the last bell faded, the quiet gave way to a roar of "only one Ricky Hatton".
The competition within the ring reflected the feeling. The eventual winner was floored by a left hook flush on the face in the second round, standing uncertainly to his feet on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up soon, noticeable damage forming under his face as the duo landed hard punches.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the decision with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were persistent and brave, all the qualities their hero Hatton embodied.
