British light heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Zach Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker landed significant shots, landing some telling individual punches in a match where the styles didn't complement each other, with excessive holding marring the fight.
The officials scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor said in the ring as he was faced negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, deserved the victory but the contender endured the second loss of his 28-match professional journey.
The Champion Secures Victory in Dull Encounter
The fighter's advancement has stalled in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a last chance - under fresh management.
He began cautiously, with both competitors prepared to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to barely take the cleaner rounds.
The audience's tolerance diminished amid regular grabbing, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding all-Manchester fight to win the European light heavyweight belt.
The stadium became quiet for a profoundly moving memorial ahead of the contest as 10 bells rang in remembrance of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound ended, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The competition during the bout matched the feeling. The eventual winner was floored by a significant blow flush on the face in the initial stage, rising shakily to his position on the eight count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was damaged soon, noticeable damage forming under his face as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur barely secured the result with official scores of even score, narrow lead and wider gap, but the two athletes were persistent and brave, exactly what their hero the legend embodied.
