UK light heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a fight where the styles didn't complement each other, with constant grabbing marring the contest.
The officials scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They prevail," Buatsi commented in the ring as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Various analysts felt Parker, 31, deserved the decision but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Claims Win in Slow Affair
Buatsi's progress has slowed in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter ready to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner stanzas.
The crowd's patience diminished because of constant clinching, with the challenger going down several times during clinches.
The underdog ended stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding all-Manchester contest to secure the European light heavyweight title.
The arena grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's death. As the last bell faded, the stillness gave way to a roar of "there's only one Ricky Hatton".
The fighting inside the ropes matched the sentiment. Arthur was floored by a powerful punch squarely on the chin in the early round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was damaged quickly, heavy swelling appearing under his orbital area as the fighters exchanged punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the decision with scoring tallies of draw, 115-113 and 115-112, but both fighters were persistent and brave, exactly what their idol Hatton stood for.
