UK light-heavyweight Buatsi made a winning yet unimpressive start to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive 10-round contest at the boxing arena.
But the challenger landed significant shots, landing some telling individual punches in a match where the styles never quite clicked, with constant grabbing disrupting the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best display but how do champions respond? They succeed," Buatsi commented in the arena as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, deserved the victory but the British boxer experienced the second setback of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Affair
The fighter's advancement has plateaued in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He started carefully, with both competitors willing to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who utilized a tight guard to narrowly win the cleaner stanzas.
The audience's tolerance wore thin amid constant clinching, with Parker slipping to the canvas several times during grappling.
Parker ended stronger, but several stanzas were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester fight to secure the continental championship.
The stadium grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony sounded in memory of the city's native Ricky Hatton, who died 46 years old in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the final chime faded, the quiet gave way to a chant of "there's only one Ricky Hatton".
The action within the ring matched the feeling. The eventual winner was floored by a powerful punch squarely on the jaw in the early round, standing uncertainly to his footing on the eight count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was damaged early, heavy swelling developing under his orbital area as the fighters exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of 114-114, close margin and 115-112, but both fighters were persistent and brave, exactly what their hero Hatton embodied.
