UK light heavyweight Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a points victory over Zach Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a contest where the techniques failed to mesh, with frequent clinching disrupting the contest.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the arena as he was greeted with negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer endured the second loss of his 28-match professional journey.
The Champion Earns Win in Uneventful Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - perhaps in what feels like a last chance - under fresh management.
He started carefully, with neither fighter willing to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who utilized a solid defense to edge the cleaner stanzas.
The spectators' acceptance wore thin amid regular grabbing, with the challenger falling to the floor multiple occasions during grappling.
The underdog closed well, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a outstanding all-Manchester fight to win the continental belt.
The stadium grew hushed for a profoundly moving homage prior to the bout as 10 bells rang in memory of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the location since Hatton's death. As the concluding sound ended, the stillness transitioned to a chant of "only one Ricky Hatton".
The competition within the ring matched the emotion. Arthur was floored by a left hook flush on the jaw in the initial stage, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured soon, heavy swelling developing under his orbital area as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over twelve rounds.
The victor edged the verdict with official scores of draw, 115-113 and wider gap, but the two athletes were determined and courageous, everything their inspiration Hatton embodied.
