British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close 10-round fight at the boxing arena.
But the challenger had his moments, delivering effective single shots in a contest where the styles failed to mesh, with excessive holding marring the fight.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi commented in the arena as he was met by boos from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Various analysts believed the challenger, 31, should have been awarded the decision but the contender suffered the second loss of his 28-match career.
The Champion Earns Victory in Uneventful Affair
Buatsi's progress has plateaued in recent years through management issues and extended periods of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who worked behind a protective stance to edge the better rounds.
The audience's tolerance diminished because of frequent holding, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to secure the European light heavyweight title.
The stadium grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony rang in memory of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the final chime ended, the silence became a chant of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was floored by a powerful punch squarely on the face in the early round, rising shakily to his footing on the count of eight.
But the traditional red equipment of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was damaged early, noticeable damage appearing under his eye as the pair traded hard punches.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the action seldom decreased over the full distance.
Arthur narrowly won the result with judges scorecards of draw, narrow lead and 115-112, but each competitor were persistent and brave, all the qualities their idol Hatton represented.
