UK light heavyweight Buatsi made a winning yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a competitive ten-round fight at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a fight where the techniques didn't complement each other, with frequent clinching marring the action.
Two judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi commented in the ringside as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Various analysts believed the challenger, 31, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-bout career.
The Champion Secures Win in Uneventful Affair
The fighter's advancement has stalled in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to take risks in a cagey opening few rounds.
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 participating in his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to edge the more effective rounds.
The audience's tolerance decreased because of regular grabbing, with Parker falling to the floor several times during clinches.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a excellent all-Manchester battle to secure the European light-heavyweight championship.
The arena grew hushed for a profoundly moving memorial prior to the bout as 10 bells sounded in remembrance of Manchester's own Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised match card in the location since the passing of Hatton. As the last bell ended, the quiet became a roar of "there's only one Ricky Hatton".
The competition within the ring matched the emotion. The eventual winner was knocked down by a powerful punch flush on the face in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured soon, noticeable damage appearing under his face as the pair exchanged hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the decision with scoring tallies of draw, 115-113 and 115-112, but both fighters were persistent and courageous, everything their idol Hatton stood for.
