British light heavyweight Buatsi made a winning yet unimpressive debut to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close 10-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a fight where the techniques never quite clicked, with constant grabbing disrupting the contest.
Two judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They win," Buatsi said in the ringside as he was faced disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Various analysts felt the challenger, 31, deserved the victory but the contender endured the second defeat of his 28-fight career.
The Champion Earns Victory in Uneventful Match
The fighter's advancement has slowed in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter ready to take risks in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who operated with a solid defense to barely take the more effective stanzas.
The audience's tolerance decreased amid constant clinching, with the challenger falling to the floor various instances during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional homage ahead of the contest as 10 bells rang in honor of local hero Ricky Hatton, who died aged 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the last bell disappeared, the quiet became a shout of "there's only one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was floored by a left hook flush on the jaw in the second round, rising shakily to his footing on the referee's count.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up early, heavy swelling appearing under his face as the pair traded hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the verdict with scoring tallies of even score, 115-113 and wider gap, but the two athletes were determined and full of heart, everything their inspiration Hatton represented.
