UK light heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the styles failed to mesh, with excessive holding spoiling the fight.
Two judges judged it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest display but what do champions do? They win," the victor stated in the arena as he was faced negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter experienced the second setback of his 28-fight professional journey.
Buatsi Earns Victory in Uneventful Affair
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second appearance at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who utilized a tight guard to narrowly win the more effective stanzas.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor various instances during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight belt.
The arena grew hushed for a extremely emotional tribute prior to the bout as 10 bells rang in remembrance of local hero Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound faded, the silence gave way to a roar of "there's only one Hatton".
The action inside the ropes matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the early round, rising shakily to his footing on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured early, noticeable damage forming under his eye as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the verdict with judges scorecards of 114-114, close margin and clearer margin, but both fighters were determined and courageous, everything their inspiration the legend embodied.
