British light-heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a points victory over Zach Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round contest at the Manchester venue.
But the challenger had his moments, delivering effective individual punches in a match where the approaches didn't complement each other, with constant grabbing spoiling the fight.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They succeed," the victor said in the ring as he was faced negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Various analysts believed the challenger, 31, should have been awarded the win but the contender experienced the second loss of his 28-match career.
The Champion Secures Triumph in Dull Affair
The fighter's advancement has slowed in recent years through management issues and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who worked behind a solid defense to narrowly win the better rounds.
The audience's tolerance diminished during frequent holding, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but several stanzas were difficult to score. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to capture the European light heavyweight championship.
The venue grew hushed for a extremely emotional tribute before the fight as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime faded, the silence gave way to a shout of "only one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was knocked down by a left hook directly to the chin in the early round, rising shakily to his position on the referee's count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising appearing under his eye as the pair traded hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur narrowly won the result with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were determined and courageous, everything their inspiration Hatton embodied.
