UK light-heavyweight Buatsi earned a win yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a fight where the techniques didn't complement each other, with constant grabbing spoiling the action.
Two judges judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They win," Buatsi stated in the ring as he was faced negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Various analysts thought the challenger, 31, should have been awarded the decision but the Derbyshire fighter experienced the second setback of his 28-match career.
Buatsi Earns Triumph in Slow Match
His career development has slowed in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter ready to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the better stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light heavyweight championship.
The venue grew hushed for a extremely emotional tribute prior to the bout as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound disappeared, the silence became a roar of "there's only one Ricky Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, getting up unsteadily to his position on the referee's count.
But the red-colored attire of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was marked up quickly, significant bruising forming under his orbital area as the pair exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over twelve rounds.
The victor narrowly won the decision with judges scorecards of 114-114, narrow lead and clearer margin, but each competitor were relentless and full of heart, exactly what their idol Hatton represented.
