British light heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a contest where the styles didn't complement each other, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what do champions do? They succeed," the victor commented in the arena as he was greeted with negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts felt Parker, 31, should have been awarded the victory but the British boxer endured the second setback of his 28-bout professional journey.
The Champion Secures Win in Uneventful Match
His career development has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the cleaner stanzas.
The spectators' acceptance diminished because of frequent holding, with the challenger going down various instances during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a outstanding all-Manchester fight to win the European light heavyweight title.
The venue grew hushed for a profoundly moving memorial before the fight as a bell ceremony tolled in honor of Manchester's own Hatton, who died at age 46 in September.
It was the premier televised match card in the location since the passing of Hatton. As the concluding sound faded, the stillness gave way to a roar of "only one Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was knocked down by a powerful punch flush on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was marked up quickly, heavy swelling developing under his orbital area as the fighters traded punishing blows.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor edged the decision with scoring tallies of even score, narrow lead and 115-112, but the two athletes were relentless and courageous, everything their idol Hatton represented.
