British light heavyweight Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round fight at the boxing arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the approaches never quite clicked, with constant grabbing marring the fight.
The scoring judges had it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," Buatsi said in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second setback of his 28-bout career.
The Champion Earns Win in Slow Affair
Buatsi's progress has slowed in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a last chance - under fresh management.
He began cautiously, with both competitors ready to take risks in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the cleaner stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
The underdog ended stronger, but multiple periods were difficult to score. However, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester battle to win the continental belt.
The venue grew hushed for a deeply poignant homage before the fight as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime disappeared, the silence gave way to a roar of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the feeling. Arthur was knocked down by a left hook flush on the jaw in the early round, getting up unsteadily to his position on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, noticeable damage developing under his face as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur edged the result with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were relentless and full of heart, exactly what their hero Hatton embodied.
