British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a match where the styles never quite clicked, with frequent clinching spoiling the contest.
The officials judged it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but what do champions do? They prevail," the victor commented in the arena as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought the challenger, thirty-one, deserved the decision but the British boxer endured the second setback of his 28-bout professional journey.
The Champion Earns Triumph in Slow Affair
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to barely take the better rounds.
The crowd's patience decreased during regular grabbing, with Parker slipping to the canvas several times during grappling.
Parker ended stronger, but multiple periods were hard to judge. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight championship.
The venue grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony rang in honor of local hero Hatton, who succumbed aged 46 in recently.
It was the premier televised fight night in the city since the passing of Hatton. As the concluding sound disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The competition during the bout reflected the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up early, significant bruising developing under his eye as the pair exchanged powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur narrowly won the result with official scores of draw, close margin and wider gap, but both fighters were determined and full of heart, exactly what their inspiration Hatton represented.
