UK light-heavyweight Buatsi made a winning yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a fight where the techniques failed to mesh, with excessive holding spoiling the action.
The officials judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi said in the ring as he was met by negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, deserved the victory but the British boxer experienced the second loss of his 28-bout professional journey.
Buatsi Earns Triumph in Dull Encounter
The fighter's advancement has plateaued in recent times through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers willing to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to edge the better periods.
The spectators' acceptance wore thin because of frequent holding, with Parker going down various instances during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a terrific local rivalry fight to win the European light-heavyweight belt.
The venue grew hushed for a extremely emotional homage prior to the bout as a bell ceremony sounded in honor of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised match card in the city since Hatton's passing. As the concluding sound faded, the silence gave way to a roar of "only one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was knocked down by a powerful punch squarely on the chin in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was damaged quickly, heavy swelling appearing under his eye as the pair traded powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor edged the verdict with judges scorecards of even score, close margin and 115-112, but both fighters were persistent and brave, exactly what their inspiration the legend stood for.
