British light heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But the challenger found success, landing some telling individual punches in a fight where the techniques didn't complement each other, with excessive holding spoiling the contest.
The scoring judges had it 96-94 to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They succeed," the victor said in the arena as he was faced boos from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, merited the decision but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
Buatsi Secures Triumph in Slow Match
Buatsi's progress has plateaued in recent years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a last chance - under recent support.
He opened tentatively, with the two boxers ready to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to barely take the more effective rounds.
The crowd's patience wore thin amid constant clinching, with Parker falling to the floor several times during close exchanges.
Parker ended stronger, but many rounds were difficult to score. However, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a excellent all-Manchester contest to secure the continental belt.
The arena grew hushed for a extremely emotional memorial before the fight as 10 bells sounded in remembrance of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the last bell disappeared, the silence gave way to a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the emotion. Arthur was knocked down by a powerful punch flush on the chin in the early round, rising shakily to his feet on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured early, heavy swelling forming under his orbital area as the fighters landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the verdict with judges scorecards of 114-114, close margin and 115-112, but each competitor were determined and full of heart, everything their inspiration the legend embodied.
