UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They succeed," Buatsi commented in the ringside as he was greeted with boos from the audience.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th career victory.
Various analysts thought the challenger, 31, deserved the win but the Derbyshire fighter endured the second defeat of his 28-match career.
The Champion Secures Win in Dull Affair
Buatsi's progress has slowed in recent times through management issues and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers ready to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to barely take the more effective stanzas.
The spectators' acceptance decreased during constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent local rivalry battle to secure the European light-heavyweight belt.
The arena grew hushed for a deeply poignant memorial before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who passed away 46 years old in recently.
It was the premier televised match card in the urban center since Hatton's death. As the last bell ended, the silence gave way to a chant of "there's just one Hatton".
The competition inside the ropes reflected the emotion. Arthur was floored by a significant blow flush on the face in the early round, rising shakily to his feet on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up early, heavy swelling forming under his orbital area as the fighters exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor narrowly won the verdict with judges scorecards of 114-114, close margin and 115-112, but the two athletes were relentless and full of heart, everything their inspiration the legend represented.
