UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a fight where the techniques didn't complement each other, with excessive holding spoiling the contest.
Two judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor commented in the arena as he was greeted with boos from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the victory but the contender suffered the second loss of his 28-bout career.
Buatsi Secures Win in Uneventful Match
His career development has plateaued in recent times through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers prepared to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who operated with a tight guard to narrowly win the better stanzas.
The audience's tolerance diminished because of regular grabbing, with the challenger falling to the floor various instances during clinches.
Parker ended stronger, but multiple periods were difficult to score. However, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to secure the European light-heavyweight belt.
The venue became quiet for a extremely emotional homage ahead of the contest as ten bells sounded in honor of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound ended, the stillness gave way to a chant of "there's only one Ricky Hatton".
The competition within the ring equaled the feeling. The eventual winner was knocked down by a significant blow squarely on the face in the second round, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured quickly, noticeable damage appearing under his eye as the pair landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur barely secured the verdict with official scores of 114-114, narrow lead and clearer margin, but each competitor were relentless and brave, everything their idol the legend embodied.
