British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The officials scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," Buatsi stated in the arena as he was faced negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th career victory.
Some ringside observers felt the challenger, 31, merited the win but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Secures Win in Dull Match
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to narrowly win the more effective periods.
The spectators' acceptance wore thin during regular grabbing, with the challenger going down various instances during grappling.
Parker closed well, but several stanzas were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding local rivalry fight to win the European light-heavyweight championship.
The stadium grew hushed for a deeply poignant homage before the fight as 10 bells rang in memory of the city's native Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell faded, the stillness gave way to a roar of "only one Hatton".
The action inside the ropes reflected the sentiment. Arthur was floored by a significant blow squarely on the chin in the initial stage, rising shakily to his feet on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged early, heavy swelling forming under his orbital area as the duo exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor edged the verdict with judges scorecards of even score, close margin and wider gap, but both fighters were relentless and courageous, exactly what their inspiration the legend embodied.
