UK light-heavyweight Buatsi earned a win yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive ten-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a match where the approaches didn't complement each other, with frequent clinching marring the contest.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi stated in the arena as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Several boxing experts thought Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
Buatsi Secures Win in Slow Match
His career development has plateaued in recent times through management issues and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter willing to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to barely take the better rounds.
The crowd's patience diminished amid frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to win the continental championship.
The stadium fell silent for a deeply poignant tribute prior to the bout as 10 bells sounded in memory of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised boxing event in the location since Hatton's death. As the final chime ended, the quiet gave way to a chant of "only one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the second round, rising shakily to his position on the count of eight.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was damaged quickly, noticeable damage appearing under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor barely secured the verdict with official scores of 114-114, narrow lead and clearer margin, but both fighters were relentless and full of heart, everything their inspiration the legend embodied.
