UK light heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight ten-round bout at the boxing arena.
But Parker found success, landing some telling single shots in a fight where the techniques didn't complement each other, with frequent clinching marring the contest.
The officials judged it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They succeed," Buatsi said in the ring as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Some ringside observers felt the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Claims Victory in Slow Encounter
Buatsi's progress has slowed in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to narrowly win the more effective rounds.
The audience's tolerance decreased because of regular grabbing, with the challenger going down multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester contest to secure the European light heavyweight title.
The arena fell silent for a profoundly moving tribute ahead of the contest as ten bells tolled in honor of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the first major televised match card in the location since Hatton's death. As the concluding sound faded, the stillness gave way to a shout of "only one Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was floored by a left hook flush on the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured early, heavy swelling developing under his orbital area as the duo traded punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the verdict with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were relentless and courageous, all the qualities their idol Hatton stood for.
