British light heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling individual punches in a match where the styles didn't complement each other, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best performance but what do champions do? They win," Buatsi commented in the ring as he was met by boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts felt the challenger, thirty-one, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Claims Victory in Slow Match
His career development has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who utilized a solid defense to narrowly win the more effective periods.
The crowd's patience diminished amid regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester contest to capture the European light-heavyweight title.
The venue became quiet for a profoundly moving homage before the fight as a bell ceremony tolled in memory of local hero Hatton, who died 46 years old in September.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime faded, the silence gave way to a roar of "there's just one Hatton".
The competition during the bout reflected the feeling. The eventual winner was knocked down by a powerful punch squarely on the chin in the second round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged early, noticeable damage appearing under his orbital area as the duo exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and 115-112, but the two athletes were relentless and brave, exactly what their idol the legend represented.
