UK light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a match where the techniques didn't complement each other, with excessive holding marring the contest.
The officials scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi commented in the ring as he was faced boos from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts believed Parker, thirty-one, deserved the decision but the contender suffered the second defeat of his 28-match career.
Buatsi Earns Triumph in Slow Affair
His career development has slowed in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who utilized a tight guard to edge the better periods.
The audience's tolerance diminished amid regular grabbing, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester fight to secure the continental title.
The venue became quiet for a profoundly moving homage before the fight as 10 bells sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell disappeared, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was floored by a powerful punch directly to the chin in the second round, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was injured soon, significant bruising forming under his eye as the pair landed powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action rarely dropped over 12 rounds.
The victor edged the verdict with judges scorecards of even score, close margin and wider gap, but each competitor were determined and full of heart, exactly what their idol Hatton represented.
