UK light-heavyweight Buatsi earned a win yet underwhelming start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a match where the techniques never quite clicked, with excessive holding spoiling the contest.
The scoring judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest performance but what do champions do? They win," the victor said in the arena as he was met by boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the decision but the British boxer endured the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Dull Encounter
His career development has stalled in recent times through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers prepared to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to barely take the more effective stanzas.
The spectators' acceptance decreased because of frequent holding, with the challenger falling to the floor several times during clinches.
Parker closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a excellent all-Manchester contest to win the European light heavyweight championship.
The arena became quiet for a deeply poignant homage ahead of the contest as ten bells rang in memory of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the last bell faded, the quiet became a shout of "there's just one Hatton".
The fighting during the bout equaled the feeling. The eventual winner was floored by a left hook flush on the face in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up soon, heavy swelling appearing under his face as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor narrowly won the decision with judges scorecards of 114-114, 115-113 and 115-112, but the two athletes were determined and brave, exactly what their idol Hatton embodied.
