British light-heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the styles failed to mesh, with excessive holding marring the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi said in the ringside as he was faced negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, merited the win but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Secures Victory in Dull Affair
Buatsi's progress has plateaued in the past few years through management issues and long spells of lack of fights.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with both competitors prepared to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the cleaner periods.
The crowd's patience wore thin during frequent holding, with the challenger going down several times during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester fight to win the continental title.
The stadium became quiet for a deeply poignant tribute ahead of the contest as ten bells rang in honor of local hero Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell ended, the quiet gave way to a roar of "there's only one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was floored by a powerful punch flush on the chin in the second round, rising shakily to his footing on the referee's count.
But the red-colored shorts of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was damaged soon, heavy swelling appearing under his face as the fighters exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the decision with official scores of 114-114, close margin and clearer margin, but both fighters were relentless and courageous, everything their hero the legend embodied.
