UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round fight at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a contest where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor said in the arena as he was faced boos from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts thought Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-fight career.
Buatsi Secures Win in Uneventful Encounter
His career development has plateaued in recent times through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He opened tentatively, with both competitors prepared to take risks in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to barely take the better rounds.
The crowd's patience diminished during regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog closed well, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Arthur defeated Bradley Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight belt.
The venue fell silent for a deeply poignant homage before the fight as 10 bells tolled in memory of local hero Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the stillness gave way to a chant of "only one Hatton".
The action within the ring equaled the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the initial stage, standing uncertainly to his feet on the eight count.
But the red-colored attire of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was damaged quickly, noticeable damage appearing under his orbital area as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur edged the decision with scoring tallies of draw, narrow lead and wider gap, but the two athletes were relentless and courageous, exactly what their hero the legend stood for.
