British light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a close 10-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the approaches didn't complement each other, with frequent clinching spoiling the action.
The officials scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They succeed," Buatsi commented in the arena as he was greeted with negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, thirty-one, merited the win but the contender endured the second loss of his 28-fight professional journey.
Buatsi Claims Triumph in Dull Affair
His career development has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to edge the more effective stanzas.
The crowd's patience diminished because of constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester battle to win the continental championship.
The venue grew hushed for a extremely emotional homage ahead of the contest as ten bells rang in memory of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound ended, the stillness transitioned to a chant of "there's just one Hatton".
The competition within the ring matched the emotion. Arthur was sent to the canvas by a powerful punch directly to the face in the initial stage, rising shakily to his footing on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged soon, heavy swelling appearing under his orbital area as the fighters landed powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor edged the decision with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were persistent and courageous, exactly what their inspiration Hatton stood for.
