UK light heavyweight Buatsi secured a victory yet underwhelming beginning to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger found success, connecting with powerful individual punches in a match where the styles didn't complement each other, with frequent clinching disrupting the fight.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi stated in the arena as he was greeted with boos from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, should have been awarded the victory but the contender experienced the second loss of his 28-match career.
Buatsi Secures Victory in Slow Affair
His career development has stalled in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the future of UK boxing, he now is positioned trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He began cautiously, with neither fighter willing to be aggressive in a calculated early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The spectators' acceptance wore thin because of constant clinching, with Parker going down several times during clinches.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester contest to win the European light-heavyweight belt.
The venue fell silent for a profoundly moving tribute before the fight as a bell ceremony tolled in memory of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell ended, the silence gave way to a shout of "there's just one Hatton".
The competition inside the ropes equaled the emotion. Arthur was floored by a left hook squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, noticeable damage appearing under his face as the duo landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the verdict with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were persistent and full of heart, all the qualities their idol the legend embodied.
