UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a close 10-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a fight where the styles never quite clicked, with constant grabbing marring the contest.
The officials had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest display but what defines champions? They win," the victor stated in the arena as he was greeted with negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers believed Parker, thirty-one, merited the win but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has stalled in the past few years through contract problems and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers ready to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the more effective rounds.
The spectators' acceptance diminished because of regular grabbing, with the challenger falling to the floor various instances during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding local rivalry battle to win the European light-heavyweight belt.
The venue became quiet for a profoundly moving tribute prior to the bout as 10 bells rang in memory of the city's native Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime disappeared, the silence became a roar of "there's just one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was injured quickly, noticeable damage forming under his eye as the pair exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over the full distance.
Arthur edged the verdict with scoring tallies of even score, close margin and wider gap, but each competitor were determined and brave, all the qualities their hero the legend stood for.
