British light-heavyweight Buatsi earned a win yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a contest where the styles failed to mesh, with excessive holding spoiling the fight.
Two judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," Buatsi stated in the ringside as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, 31, merited the win but the British boxer endured the second loss of his 28-match career.
The Champion Claims Victory in Dull Match
Buatsi's progress has slowed in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to edge the cleaner periods.
The crowd's patience wore thin amid regular grabbing, with Parker slipping to the canvas several times during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding local rivalry battle to secure the European light-heavyweight championship.
The arena grew hushed for a profoundly moving tribute ahead of the contest as 10 bells tolled in memory of the city's native Hatton, who succumbed 46 years old in last month.
It was the first major televised fight night in the location since Hatton's death. As the final chime disappeared, the silence became a roar of "there's only one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was marked up soon, significant bruising appearing under his orbital area as the pair exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
Arthur barely secured the decision with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and full of heart, everything their inspiration Hatton represented.
