UK light-heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a match where the styles failed to mesh, with frequent clinching marring the fight.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the arena as he was greeted with boos from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the decision but the contender suffered the second defeat of his 28-bout professional journey.
The Champion Claims Victory in Uneventful Encounter
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers ready to take risks in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who utilized a protective stance to edge the more effective stanzas.
The crowd's patience diminished during constant clinching, with Parker going down several times during clinches.
Parker ended stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Inspires Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester battle to win the continental championship.
The stadium became quiet for a profoundly moving homage ahead of the contest as a bell ceremony rang 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 passing. As the concluding sound disappeared, the stillness gave way to a chant of "there's only one Hatton".
The action during the bout matched the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was damaged early, significant bruising developing under his eye as the fighters landed punishing blows.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the decision with scoring tallies of draw, 115-113 and 115-112, but each competitor were determined and courageous, everything their hero Hatton represented.
