British light-heavyweight Buatsi earned a win yet unimpressive debut to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive 10-round contest at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the techniques never quite clicked, with excessive holding disrupting the contest.
The officials judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They prevail," the victor commented in the arena as he was met by disapproval from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to earn a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the decision but the contender endured the second setback of his 28-bout professional journey.
Buatsi Earns Triumph in Dull Match
The fighter's advancement has stalled in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who operated with a solid defense to barely take the better periods.
The crowd's patience decreased because of regular grabbing, with Parker falling to the floor several times during clinches.
Parker closed well, but several stanzas were difficult to score. Although, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the supporting bout, Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester contest to secure the European light heavyweight championship.
The venue became quiet for a deeply poignant memorial before the fight as 10 bells tolled in memory of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised boxing event in the location since Hatton's passing. As the final chime disappeared, the stillness gave way to a chant of "only one Hatton".
The action during the bout reflected the feeling. The eventual winner was sent to the canvas by a left hook squarely on the face in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was injured soon, heavy swelling appearing under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor edged the decision with judges scorecards of 114-114, close margin and clearer margin, but both fighters were persistent and full of heart, all the qualities their idol Hatton embodied.
