British light heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a match where the approaches failed to mesh, with frequent clinching spoiling the action.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," the victor stated in the ringside as he was faced negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, merited the decision but the British boxer experienced the second loss of his 28-bout professional journey.
The Champion Claims Win in Uneventful Affair
His career development has plateaued in recent times through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under fresh management.
He started carefully, with neither fighter willing to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to edge the better stanzas.
The audience's tolerance decreased during frequent holding, with the challenger falling to the floor several times during grappling.
The underdog closed well, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester contest to capture the continental belt.
The arena fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in memory of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell ended, the silence became a chant of "only one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was sent to the canvas by a left hook squarely on the jaw in the early round, standing uncertainly to his position on the eight count.
But the red-colored attire of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up quickly, noticeable damage developing under his orbital area as the pair landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the result with judges scorecards of even score, narrow lead and clearer margin, but both fighters were persistent and brave, everything their hero the legend stood for.
