UK light heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a close 10-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a match where the styles never quite clicked, with constant grabbing disrupting the contest.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the arena as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed Parker, 31, merited the victory but the British boxer suffered the second setback of his 28-match career.
The Champion Claims Victory in Slow Encounter
His career development has stalled in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to barely take the cleaner periods.
The crowd's patience decreased because of constant clinching, with Parker going down various instances during clinches.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific local rivalry fight to capture the continental championship.
The arena grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime ended, the stillness became a shout of "there's just one Ricky Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was floored by a powerful punch squarely on the jaw in the second round, standing uncertainly to his feet on the eight count.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was marked up soon, noticeable damage forming under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the decision with scoring tallies of even score, close margin and clearer margin, but the two athletes were relentless and courageous, everything their inspiration Hatton represented.
