UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a competitive ten-round contest at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the approaches never quite clicked, with frequent clinching marring the action.
The officials had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my finest display but what defines champions? They succeed," the victor stated in the ringside as he was greeted with negative reactions from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, deserved the win but the Derbyshire fighter endured the second setback of his 28-bout career.
The Champion Secures Triumph in Uneventful Encounter
The fighter's advancement has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who utilized a tight guard to edge the better stanzas.
The crowd's patience wore thin amid constant clinching, with Parker falling to the floor several times during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding local rivalry contest to secure the European light-heavyweight championship.
The arena became quiet for a profoundly moving tribute before the fight as 10 bells tolled in honor of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the premier televised match card in the location since Hatton's death. As the final chime faded, the stillness transitioned to a roar of "there's just one Hatton".
The action during the bout equaled the feeling. The eventual winner was sent to the canvas by a left hook flush on the jaw in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was damaged quickly, heavy swelling appearing under his eye as the fighters exchanged powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over the full distance.
Arthur edged the result with official scores of draw, narrow lead and clearer margin, but the two athletes were determined and brave, all the qualities their hero Hatton represented.
