British light heavyweight Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a contest where the styles failed to mesh, with frequent clinching disrupting the fight.
Two judges judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the ring as he was faced disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts thought the challenger, 31, merited the win but the British boxer suffered the second loss of his 28-fight career.
The Champion Earns Triumph in Uneventful Encounter
The fighter's advancement has slowed in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who operated with a tight guard to edge the more effective periods.
The crowd's patience diminished during regular grabbing, with Parker going down multiple occasions during close exchanges.
The underdog closed well, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to secure the European light heavyweight title.
The venue became quiet for a deeply poignant memorial prior to the bout as 10 bells sounded in honor of local hero Hatton, who succumbed 46 years old in recently.
It was the premier televised fight night in the urban center since Hatton's death. As the final chime faded, the stillness became a shout of "there's just one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up quickly, noticeable damage forming under his face as the fighters landed punishing blows.
Rea, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the decision with official scores of 114-114, close margin and wider gap, but both fighters were persistent and brave, all the qualities their idol the legend represented.
