UK light heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a match where the techniques failed to mesh, with frequent clinching spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best display but how do champions respond? They prevail," the victor commented in the ringside as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Various analysts felt Parker, thirty-one, deserved the win but the British boxer suffered the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Slow Match
Buatsi's progress has stalled in recent times through management issues and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with the two boxers prepared to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to narrowly win the better periods.
The audience's tolerance wore thin during frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
The underdog closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester contest to secure the European light-heavyweight title.
The stadium fell silent for a profoundly moving memorial ahead of the contest as ten bells rang in memory of local hero Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the location since Hatton's passing. As the final chime disappeared, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The fighting inside the ropes equaled the feeling. Arthur was knocked down by a significant blow directly to the chin in the second round, rising shakily to his feet on the eight count.
But the red-colored attire of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was injured early, significant bruising appearing under his face as the duo exchanged powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over the full distance.
The victor barely secured the verdict with official scores of 114-114, close margin and 115-112, but each competitor were persistent and full of heart, everything their hero the legend embodied.
