UK light heavyweight Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques didn't complement each other, with constant grabbing spoiling the fight.
Two judges judged it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," the victor commented in the arena as he was met by disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Various analysts thought the challenger, thirty-one, deserved the win but the contender suffered the second setback of his 28-match professional journey.
The Champion Claims Win in Slow Affair
His career development has plateaued in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the next star of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under recent support.
He began cautiously, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who operated with a protective stance to barely take the more effective periods.
The audience's tolerance wore thin during frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but several stanzas were hard to judge. However, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a excellent all-Manchester battle to capture the European light heavyweight championship.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as 10 bells tolled in memory of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the concluding sound disappeared, the quiet became a roar of "only one Hatton".
The action inside the ropes matched the sentiment. The eventual winner was floored by a left hook squarely on the chin in the second round, standing uncertainly to his position on the referee's count.
But the red-colored attire of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was injured quickly, heavy swelling appearing under his face as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over 12 rounds.
The victor barely secured the verdict with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were persistent and full of heart, everything their inspiration Hatton embodied.
