UK light-heavyweight Buatsi made a winning yet unimpressive debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a close 10-round fight at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a contest where the styles didn't complement each other, with excessive holding disrupting the contest.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best showing but how do champions respond? They win," the victor commented in the ring as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, thirty-one, deserved the win but the contender endured the second loss of his 28-match professional journey.
The Champion Earns Victory in Dull Encounter
Buatsi's progress has slowed in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to narrowly win the more effective rounds.
The spectators' acceptance wore thin because of frequent holding, with the challenger falling to the floor several times during grappling.
The underdog closed well, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester contest to win the European light heavyweight championship.
The stadium grew hushed for a deeply poignant memorial before the fight as 10 bells sounded in honor of the city's native Ricky Hatton, who died 46 years old in last month.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound disappeared, the silence became a shout of "only one Ricky Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was damaged early, significant bruising developing under his orbital area as the fighters landed punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of even score, close margin and clearer margin, but both fighters were persistent and full of heart, all the qualities their hero the legend embodied.
