UK light heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round contest at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a contest where the approaches never quite clicked, with constant grabbing marring the fight.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"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 British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th career victory.
Some ringside observers thought the challenger, thirty-one, should have been awarded the decision but the British boxer experienced the second setback of his 28-match professional journey.
Buatsi Claims Victory in Uneventful Match
The fighter's advancement has plateaued in the past few years through contract problems and long spells of inactivity.
Previously predicted as the future of UK 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 neither fighter willing to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was participating in his second fight at light-heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who utilized a tight guard to narrowly win the more effective periods.
The spectators' acceptance wore thin amid constant clinching, with Parker slipping to the canvas various instances during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a outstanding all-Manchester battle to secure the European light-heavyweight championship.
The arena fell silent for a extremely emotional memorial prior to the bout as a bell ceremony sounded in honor of the city's native Hatton, who died at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound disappeared, the silence gave way to a chant of "there's just one Ricky Hatton".
The action within the ring reflected the feeling. Arthur was floored by a significant blow flush on the jaw in the early round, rising shakily to his feet on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged soon, noticeable damage developing under his orbital area as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over twelve rounds.
The victor barely secured the decision with scoring tallies of 114-114, close margin and 115-112, but the two athletes were relentless and full of heart, exactly what their hero the legend stood for.
