British light heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the styles failed to mesh, with constant grabbing disrupting the fight.
The officials scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They prevail," the victor said in the ring as he was met by boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Various analysts thought Parker, 31, deserved the decision but the British boxer endured the second defeat of his 28-bout career.
The Champion Earns Victory in Slow Encounter
His career development has plateaued in the past few years through management issues and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He started carefully, with both competitors willing to take risks in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to barely take the better periods.
The spectators' acceptance decreased amid frequent holding, with the challenger falling to the floor various instances during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester contest to secure the European light heavyweight belt.
The venue became quiet for a extremely emotional homage before the fight as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the location since the passing of Hatton. As the concluding sound ended, the silence transitioned to a roar of "only one Ricky Hatton".
The competition during the bout equaled the feeling. Arthur was sent to the canvas by a left hook directly to the face in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was injured soon, heavy swelling appearing under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the action rarely dropped over the full distance.
The victor narrowly won the result with scoring tallies of 114-114, 115-113 and wider gap, but both fighters were persistent and courageous, exactly what their idol the legend stood for.
