UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the techniques didn't complement each other, with excessive holding spoiling the contest.
The officials had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They win," Buatsi commented in the arena as he was met by boos from the audience.
The British fighter returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th career victory.
Various analysts felt Parker, 31, merited the win but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Claims Win in Uneventful Match
Buatsi's progress has plateaued in recent years through management issues and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who operated with a protective stance to edge the better stanzas.
The audience's tolerance diminished during constant clinching, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the supporting bout, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light-heavyweight championship.
The arena grew hushed for a profoundly moving memorial before the fight as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the urban center since Hatton's death. As the last bell ended, the stillness became a shout of "only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the second round, rising shakily to his feet on the count of eight.
But the red-colored equipment of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was damaged quickly, heavy swelling forming under his orbital area as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over twelve rounds.
The victor edged the verdict with official scores of draw, 115-113 and wider gap, but each competitor were determined and courageous, exactly what their idol Hatton represented.
