UK light heavyweight Buatsi secured a victory yet disappointing debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a tight ten-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the techniques failed to mesh, with frequent clinching disrupting the fight.
The scoring judges judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They succeed," the victor stated in the arena as he was greeted with disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, should have been awarded the win but the British boxer endured the second defeat of his 28-fight career.
Buatsi Earns Victory in Uneventful Affair
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter prepared to take risks in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who worked behind a solid defense to narrowly win the cleaner stanzas.
The spectators' acceptance wore thin because of frequent holding, with Parker going down several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Battle
In the co-main event, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to capture the European light heavyweight title.
The stadium fell silent for a extremely emotional homage ahead of the contest as ten bells rang in honor of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised match card in the city since Hatton's passing. As the final chime ended, the silence gave way to a roar of "there's just one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured early, heavy swelling forming under his face as the fighters exchanged hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor narrowly won the verdict with judges scorecards of draw, close margin and 115-112, but both fighters were persistent and courageous, exactly what their idol the legend embodied.
