British light heavyweight Buatsi made a winning yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a fight where the techniques failed to mesh, with constant grabbing marring the contest.
Two judges had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They win," Buatsi commented in the ring as he was faced boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, thirty-one, should have been awarded the decision but the contender suffered the second setback of his 28-bout professional journey.
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who operated with a protective stance to narrowly win the more effective rounds.
The crowd's patience decreased during frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light-heavyweight championship.
The stadium grew hushed for a profoundly moving homage ahead of the contest as 10 bells sounded in remembrance of local hero Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a roar of "there's only one Hatton".
The competition within the ring equaled the emotion. Arthur was sent to the canvas by a powerful punch directly to the jaw in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was marked up early, significant bruising forming under his eye as the duo landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of even score, narrow lead and 115-112, but both fighters were persistent and courageous, everything their inspiration Hatton represented.