British light heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round bout at the boxing arena.
But Parker had his moments, delivering effective single shots in a contest where the approaches didn't complement each other, with constant grabbing spoiling the contest.
The officials scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They succeed," the victor said in the ringside as he was met by boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought Parker, thirty-one, merited the victory but the contender endured the second defeat of his 28-fight professional journey.
The Champion Claims Victory in Slow Affair
His career development has plateaued in recent times through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat 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, causing problems for the favorite, who worked behind a solid defense to barely take the better rounds.
The spectators' acceptance decreased amid regular grabbing, with the challenger going down several times during grappling.
The underdog ended stronger, but multiple periods were difficult to score. Although, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Homage Fuels All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light heavyweight championship.
The venue grew hushed for a deeply poignant homage prior to the bout as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the city since Hatton's death. As the last bell ended, the quiet gave way to a chant of "there's only one Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a left hook directly to the chin in the second round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged early, significant bruising appearing under his face as the duo exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
The victor barely secured the result with judges scorecards of draw, 115-113 and wider gap, but each competitor were determined and courageous, all the qualities their idol Hatton represented.
