UK light heavyweight Buatsi made a winning yet underwhelming debut to life under new promoters 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 fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a contest where the styles failed to mesh, with frequent clinching disrupting the fight.
The officials scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They succeed," Buatsi said in the ringside as he was met by negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts believed Parker, 31, deserved the victory but the contender experienced the second setback of his 28-fight professional journey.
Buatsi Secures Victory in Uneventful Match
The fighter's advancement has stalled in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with neither fighter willing to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to barely take the more effective stanzas.
The audience's tolerance wore thin during frequent holding, with the challenger slipping to the canvas various instances during clinches.
Parker closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding local rivalry battle to capture the European light heavyweight belt.
The venue grew hushed for a extremely emotional homage before the fight as 10 bells sounded in memory of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's death. As the last bell ended, the stillness gave way to a shout of "there's just one Hatton".
The action within the ring matched the emotion. Arthur was sent to the canvas by a left hook squarely on the chin in the second round, rising shakily to his footing on the count of eight.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was injured quickly, heavy swelling appearing under his eye as the fighters traded hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor edged the verdict with scoring tallies of draw, narrow lead and clearer margin, but each competitor were persistent and full of heart, everything their hero the legend stood for.
