UK light-heavyweight Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round contest at the Manchester venue.
But Parker found success, landing some telling clean blows in a match where the approaches didn't complement each other, with excessive holding spoiling the fight.
Two judges scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what do champions do? They win," Buatsi commented in the ring as he was faced boos from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Several boxing experts thought Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-match career.
The Champion Claims Win in Slow Encounter
His career development has stalled in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a last chance - under fresh management.
He began cautiously, with both competitors ready to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the more effective stanzas.
The spectators' acceptance diminished amid frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, former world champion Carl Frampton - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry contest to capture the European light heavyweight belt.
The stadium became quiet for a extremely emotional homage ahead of the contest as a bell ceremony tolled in memory of local hero Hatton, who succumbed aged 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the concluding sound ended, the silence became a chant of "there's only one Hatton".
The competition during the bout reflected the emotion. Arthur was knocked down by a left hook flush on the chin in the early round, standing uncertainly to his position on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was injured quickly, heavy swelling forming under his orbital area as the duo exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
Arthur edged the result with scoring tallies of even score, narrow lead and clearer margin, but each competitor were relentless and brave, exactly what their idol Hatton stood for.
