British light heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a contest where the techniques never quite clicked, with frequent clinching marring the fight.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the ringside as he was faced boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the win but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Match
Buatsi's progress has plateaued in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the future of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He started carefully, with the two boxers ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to narrowly win the cleaner periods.
The crowd's patience diminished because of constant clinching, with the challenger going down multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester fight to capture the European light-heavyweight championship.
The venue became quiet for a profoundly moving homage ahead of the contest as a bell ceremony rang in memory of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the premier televised match card in the location since Hatton's passing. As the final chime ended, the silence transitioned to a chant of "there's just one Hatton".
The action during the bout reflected the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the face in the early round, getting up unsteadily to his footing on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up soon, noticeable damage developing under his face as the fighters landed hard punches.
The challenger, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur narrowly won the result with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were determined and brave, exactly what their idol the legend stood for.
