UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive ten-round bout at the boxing arena.
But Parker had his moments, landing some telling individual punches in a match where the approaches never quite clicked, with excessive holding spoiling the fight.
The officials judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They win," Buatsi said in the ring as he was greeted with disapproval from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers felt Parker, 31, merited the victory but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Earns Victory in Slow Match
His career development has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to realize his potential - maybe in what feels like a final roll of the dice - under recent support.
He opened tentatively, with both competitors prepared to overcommit in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who worked behind a protective stance to edge the cleaner rounds.
The spectators' acceptance diminished because of frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Arthur beat Rea by divided decision in a terrific all-Manchester fight to win the continental championship.
The arena became quiet for a profoundly moving memorial before the fight as a bell ceremony rang in honor of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime faded, the stillness gave way to a chant of "only one Ricky Hatton".
The fighting within the ring matched the emotion. Arthur was sent to the canvas by a significant blow squarely on the chin in the early round, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged quickly, significant bruising appearing under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor narrowly won the verdict with scoring tallies of even score, 115-113 and wider gap, but each competitor were persistent and courageous, everything their hero the legend represented.
