British light heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the approaches didn't complement each other, with constant grabbing marring the fight.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what do champions do? They prevail," Buatsi stated in the ring as he was faced negative reactions from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers felt Parker, 31, should have been awarded the victory but the Derbyshire fighter experienced the second defeat of his 28-bout career.
The Champion Earns Triumph in Dull Match
The fighter's advancement has slowed in the past few years through contract problems and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to edge the cleaner periods.
The crowd's patience wore thin during regular grabbing, with Parker going down several times during clinches.
Parker closed well, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding Manchester vs Manchester contest to win the continental belt.
The arena became quiet for a extremely emotional tribute prior to the bout as ten bells rang in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound ended, the quiet transitioned to a chant of "only one Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a left hook directly to the chin in the initial stage, rising shakily to his footing on the eight count.
But the traditional red attire of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was injured early, noticeable damage developing under his face as the duo exchanged hard punches.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor barely secured the verdict with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were determined and brave, exactly what their inspiration Hatton embodied.
