UK light-heavyweight Buatsi earned a win yet underwhelming debut to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the styles never quite clicked, with frequent clinching disrupting the fight.
Two judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," Buatsi said in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt Parker, thirty-one, should have been awarded the victory but the British boxer endured the second loss of his 28-match career.
Buatsi Secures Triumph in Uneventful Encounter
The fighter's advancement has slowed in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to barely take the better rounds.
The spectators' acceptance diminished amid constant clinching, with the challenger going down multiple occasions during grappling.
Parker closed well, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a terrific local rivalry battle to secure the continental title.
The stadium grew hushed for a profoundly moving homage prior to the bout as ten bells sounded in honor of local hero Hatton, who passed away at age 46 in September.
It was the first major televised boxing event in the location since Hatton's death. As the last bell faded, the silence became a chant of "only one Hatton".
The competition during the bout reflected the feeling. Arthur was floored by a left hook directly to the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up early, heavy swelling appearing under his face as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, 115-113 and wider gap, but each competitor were persistent and brave, everything their hero Hatton stood for.
