UK light-heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a unanimous decision over Zach Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a match where the techniques didn't complement each other, with frequent clinching spoiling the contest.
Two judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They prevail," Buatsi said in the ringside as he was faced negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts felt Parker, thirty-one, deserved the win but the contender experienced the second setback of his 28-fight professional journey.
The Champion Claims Win in Slow Affair
Buatsi's progress has stalled in recent years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He began cautiously, with the two boxers willing to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who utilized a solid defense to barely take the more effective rounds.
The audience's tolerance diminished amid constant clinching, with Parker going down various instances during grappling.
The underdog ended stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to secure the European light heavyweight championship.
The stadium fell silent for a extremely emotional memorial before the fight as ten bells rang in honor of local hero Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the final chime ended, the quiet transitioned to a roar of "there's only one Hatton".
The action during the bout reflected the sentiment. The eventual winner was knocked down by a left hook directly to the chin in the initial stage, getting up unsteadily to his footing on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling forming under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over the full distance.
Arthur edged the verdict with judges scorecards of 114-114, close margin and 115-112, but both fighters were relentless and courageous, exactly what their idol the legend embodied.
