UK light heavyweight Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight ten-round contest at the Manchester venue.
But Parker found success, delivering effective clean blows in a contest where the techniques didn't complement each other, with frequent clinching disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but what defines champions? They prevail," the victor commented in the ring as he was met by disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, should have been awarded the victory but the contender endured the second defeat of his 28-bout professional journey.
The Champion Claims Triumph in Slow Encounter
His career development has plateaued in recent times through contract problems and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to edge the more effective periods.
The crowd's patience diminished because of frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a terrific local rivalry battle to secure the continental title.
The venue fell silent for a deeply poignant tribute prior to the bout as 10 bells tolled in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the last bell ended, the stillness gave way to a shout of "only one Hatton".
The competition during the bout matched the feeling. Arthur was sent to the canvas by a powerful punch directly to the face in the initial stage, getting up unsteadily to his position on the eight count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured soon, significant bruising developing under his face as the duo traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of draw, narrow lead and clearer margin, but the two athletes were persistent and brave, everything their hero Hatton embodied.
