British light-heavyweight Buatsi earned a win yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the approaches failed to mesh, with frequent clinching marring the action.
The officials judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ring as he was faced negative reactions from the crowd.
The Londoner 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 the challenger, 31, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
The Champion Claims Victory in Dull Encounter
Buatsi's progress has stalled in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a cagey initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a tight guard to edge the better stanzas.
The crowd's patience diminished amid frequent holding, with the challenger going down multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester battle to capture the continental belt.
The venue grew hushed for a profoundly moving memorial before the fight as ten bells rang in memory of Manchester's own Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the final chime disappeared, the stillness became a roar of "there's only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was sent to the canvas by a significant blow flush on the jaw in the early round, rising shakily to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured soon, noticeable damage developing under his eye as the duo landed punishing blows.
Rea, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over 12 rounds.
The victor barely secured the result with scoring tallies of draw, close margin and 115-112, but both fighters were persistent and full of heart, exactly what their hero Hatton represented.
