UK light-heavyweight Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close ten-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
Two judges judged it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," the victor said in the ring as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts believed the challenger, thirty-one, should have been awarded the victory but the British boxer experienced the second loss of his 28-bout career.
Buatsi Claims Victory in Uneventful Affair
Buatsi's progress has slowed in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to barely take the better periods.
The spectators' acceptance decreased amid regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Arthur defeated Rea by close verdict in a outstanding all-Manchester battle to capture the European light heavyweight belt.
The arena grew hushed for a deeply poignant memorial ahead of the contest as 10 bells sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell disappeared, the quiet gave way to a shout of "there's just one Ricky Hatton".
The competition within the ring matched the emotion. Arthur was sent to the canvas by a powerful punch directly to the jaw in the initial stage, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was marked up soon, significant bruising forming under his eye as the pair landed hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur barely secured the decision with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were relentless and full of heart, exactly what their inspiration the legend embodied.
