British light-heavyweight Buatsi made a winning yet underwhelming start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a fight where the styles didn't complement each other, with constant grabbing marring the fight.
The officials scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They prevail," Buatsi said in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Various analysts thought Parker, 31, merited the win but the contender suffered the second setback of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Encounter
Buatsi's progress has plateaued in recent times through management issues and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective rounds.
The crowd's patience wore thin during frequent holding, with Parker going down several times during clinches.
Parker ended stronger, but several stanzas were challenging to assess. Although, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent local rivalry battle to secure the European light-heavyweight title.
The stadium became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who died aged 46 in recently.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell faded, the stillness transitioned to a chant of "only one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was floored by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged soon, noticeable damage forming under his eye as the pair traded powerful shots.
Rea, 27, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the intensity rarely dropped over the full distance.
Arthur narrowly won the decision with official scores of draw, close margin and clearer margin, but each competitor were persistent and courageous, all the qualities their hero the legend represented.
