UK light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive 10-round contest at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with constant grabbing disrupting the action.
The scoring judges had it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi commented in the arena as he was faced disapproval from the crowd.
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.
Several boxing experts thought Parker, 31, merited the victory but the Derbyshire fighter suffered the second setback of his 28-bout career.
Buatsi Claims Win in Slow Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter prepared to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner periods.
The audience's tolerance diminished amid frequent holding, with the challenger going down various instances during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a terrific local rivalry contest to capture the European light heavyweight belt.
The arena fell silent for a deeply poignant homage ahead of the contest as ten bells rang in memory of local hero Hatton, who died aged 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the last bell faded, the quiet became a roar of "there's only one Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was floored by a powerful punch directly to the chin in the early round, standing uncertainly to his position on the eight count.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, significant bruising appearing under his orbital area as the pair landed punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity rarely dropped over twelve rounds.
The victor edged the verdict with official scores of draw, narrow lead and clearer margin, but each competitor were determined and brave, all the qualities their hero Hatton represented.
