UK light-heavyweight Buatsi earned a win yet disappointing beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, landing some telling single shots in a fight where the techniques failed to mesh, with excessive holding spoiling the fight.
The officials had it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor stated in the arena as he was faced disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the victory but the contender experienced the second setback of his 28-match career.
Buatsi Claims Triumph in Dull Affair
The fighter's advancement has plateaued in recent years through management issues and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers prepared to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who utilized a tight guard to narrowly win the better rounds.
The audience's tolerance diminished because of regular grabbing, with Parker going down several times during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight championship.
The arena grew hushed for a extremely emotional memorial prior to the bout as 10 bells rang in honor of local hero Hatton, who died aged 46 in recently.
It was the premier televised match card in the city since the passing of Hatton. As the final chime ended, the stillness gave way to a shout of "only one Ricky Hatton".
The competition during the bout matched the emotion. The eventual winner was knocked down by a powerful punch directly to the jaw in the early round, rising shakily to his feet on the eight count.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged quickly, noticeable damage appearing under his eye as the pair traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur edged the result with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were determined and brave, everything their inspiration Hatton embodied.
