UK light-heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling clean blows in a match where the techniques never quite clicked, with excessive holding spoiling the action.
Two judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest display but how do champions respond? They succeed," Buatsi commented in the ringside as he was met by disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt Parker, 31, merited the win but the British boxer suffered the second loss of his 28-bout career.
Buatsi Earns Victory in Slow Match
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter ready to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent 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 made it competitive, making life awkward for Buatsi, who worked behind a tight guard to barely take the cleaner stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but several stanzas were hard to judge. However, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Fuels Manchester Derby Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester fight to win the European light-heavyweight title.
The arena fell silent for a deeply poignant memorial prior to the bout as a bell ceremony sounded in remembrance of local hero Hatton, who died 46 years old in recently.
It was the premier televised fight night in the location since Hatton's passing. As the last bell ended, the stillness became a chant of "there's just one Hatton".
The fighting within the ring reflected the emotion. Arthur was knocked down by a significant blow flush on the chin in the second round, rising shakily to his footing on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, noticeable damage developing under his orbital area as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the verdict with scoring tallies of 114-114, narrow lead and wider gap, but both fighters were determined and courageous, exactly what their inspiration the legend stood for.
