UK light heavyweight Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a contest where the approaches never quite clicked, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They win," Buatsi said in the arena as he was faced negative reactions from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, merited the win but the Derbyshire fighter endured the second setback of his 28-match professional journey.
The Champion Secures Victory in Uneventful Match
The fighter's advancement has slowed in recent times through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He started carefully, with the two boxers willing to take risks in a cagey opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who operated with a protective stance to barely take the more effective rounds.
The crowd's patience wore thin amid constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog closed well, but several stanzas were difficult to score. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester contest to win the continental belt.
The arena grew hushed for a extremely emotional tribute ahead of the contest as ten bells rang in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell disappeared, the silence transitioned to a roar of "only one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was knocked down by a powerful punch squarely on the face in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, noticeable damage appearing under his orbital area as the fighters landed hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
Arthur edged the result with judges scorecards of 114-114, 115-113 and 115-112, but each competitor were persistent and courageous, exactly what their inspiration Hatton represented.
