UK light-heavyweight Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a match where the styles didn't complement each other, with excessive holding spoiling the fight.
The officials had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what do champions do? They prevail," Buatsi said in the arena as he was faced boos from the crowd.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts felt Parker, thirty-one, merited the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
The Champion Earns Triumph in Dull Match
The fighter's advancement has plateaued in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to take risks in a cagey initial rounds.
Parker, 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 making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a protective stance to edge the cleaner periods.
The spectators' acceptance wore thin during frequent holding, with Parker falling to the floor various instances during grappling.
Parker closed well, but multiple periods were hard to judge. However, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the co-main event, Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester fight to win the European light-heavyweight championship.
The arena fell silent for a extremely emotional homage ahead of the contest as 10 bells rang in memory of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound ended, the quiet gave way to a chant of "only one Ricky Hatton".
The competition inside the ropes matched the emotion. Arthur was floored by a powerful punch squarely on the chin in the initial stage, rising shakily to his feet on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was damaged soon, noticeable damage forming under his eye as the duo traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the result with official scores of draw, close margin and wider gap, but the two athletes were persistent and brave, all the qualities their inspiration Hatton stood for.
